This'll be pretty trivial to do - I'd say it'd be useful in the same directives/sections we're providing possible values for in the editor. That is - not just options and parameters, but triggers, agent, when, post... All of those are either extension points or have a well-defined filter for determining what contents are allowed, so might as well go whole hog.

I'd consider the priority on this to be higher than Minor. It's frustrating trying to use pipeline but continually having to figure out what undocumented features are.

For anyone else ending up here from the documentation, for parameters I've found the best resource to be this chunk of source:

https://github.com/jenkinsci/job-dsl-plugin/blob/master/job-dsl-core/src/main/groovy/javaposse/jobdsl/dsl/helpers/BuildParametersContext.groovy

Yes - doc'ing the available parameters and how to's on the Pipeline syntax page would have saved much googling, trial and error, wailing, and gnashing of teeth... 

In case it helps anyone looking to use the credentials parameter in declarative (mentioned as possible using the .groovy link above), I found a couple ways to skin the cat, but this method is the most "declarative" vs. "scripted" and seems to be working.

Undoubtedly, there's something wrong with the below (for ex. I have no idea what the credentialType values should be, or if required is required ), OR it will break with the next update to any of the pipeline plugins (been burned many times these last few months), but for now, this seems to be doing the trick to help collect a Jenkins user as a parameter for a maven/artifactory/maven release pipeline I'm prototyping.  

At least it's some documentation that might be useful to someone.

#!groovy
pipeline {
    agent { label 'any' }
    
    parameters {
        choice(name: 'PerformMavenRelease', choices: 'False\nTrue', description: 'Whether or not to perform a maven release')
        credentials(name: 'CredsToUse', description: 'A user to build with', defaultValue: '', credentialType: "Username with password", required: true )
    } 
    
    environment {
        BUILD_USR_CREDS = credentials("${params.CredsToUse}")
    }           
    
    stages {
        stage ('Sync and Setup') {
            steps {    
                echo 'BUILD_USR_CREDS_USR ' + BUILD_USR_CREDS_USR
                echo 'BUILD_USR_CREDS_PSW ' + BUILD_USR_CREDS_PSW
            }    
        }        
    }
}

Is the GIT Parameters plugin supported for the pipeline jenkinsfile declarative?

Ok, I spoke a bit too soon when saying it'd be trivial. =) Gonna think on this and start poking around some possible approaches for the next couple days, we'll see what I can come up with.

Ok, starting work - added help.html for all Describable extensions in Declarative directives (i.e., agent types, when conditions, and DeclarativeOption implementations for the options directive). Sadly, post conditions aren't extensions of Describable so they can't use the same tooling as the other directives. Sigh. https://github.com/jenkinsci/pipeline-model-definition-plugin/pull/219 has the work so far.

Veeeeery preliminary asciidoc generation is up at https://github.com/jenkins-infra/pipeline-steps-doc-generator/pull/12 now. We'll see if it, y'know, works.

Code changed in jenkins
User: Andrew Bayer
Path:
pipeline-model-definition/src/main/java/org/jenkinsci/plugins/pipeline/modeldefinition/options/impl/SkipDefaultCheckout.java
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-args.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-customWorkspace.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-label.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-registryCredentialsId.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-registryUrl.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/AbstractDockerAgent/help-reuseNode.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/Any/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipeline/help-alwaysPull.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipeline/help-image.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipeline/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipelineFromDockerfile/help-additionalBuildArgs.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipelineFromDockerfile/help-dir.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipelineFromDockerfile/help-filename.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/DockerPipelineFromDockerfile/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/Label/help-customWorkspace.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/Label/help-label.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/Label/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/agent/impl/None/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/options/impl/SkipDefaultCheckout/help-skipDefaultCheckout.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/options/impl/SkipDefaultCheckout/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/options/impl/SkipDefaultCheckout/help.jelly
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/options/impl/SkipStagesAfterUnstable/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/AllOfConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/AnyOfConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/BranchConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeLogConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeSetConditional/help-caseSensitive.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeSetConditional/help-glob.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ChangeSetConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/EnvironmentConditional/help-ignoreCase.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/EnvironmentConditional/help-name.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/EnvironmentConditional/help-value.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/EnvironmentConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/ExpressionConditional/help.html
pipeline-model-definition/src/main/resources/org/jenkinsci/plugins/pipeline/modeldefinition/when/impl/NotConditional/help.html
http://jenkins-ci.org/commit/pipeline-model-definition-plugin/7b7a3c75545666fd086f1a3317e4a750d6dff8dd
Log:
Add help HTML for user-facing Describables

This will help enable INFRA-1053 (auto-generation of help for
Declarative directives, like the step documentation on jenkins.io).

So three years later we still don't have all parameters documented. Can we have at least an manual documentation of the parameters please?

Would be great for this to work, since declarative pipeline is already deprecating the use of properties

Any news on that?
As Fabian said:
>Can we have at least an manual documentation of the parameters please?