The text shown in the Snippet Generator for the parallel step explains the syntax and notes that mechanical generation of snippets is not available in this case. I am not sure what else there is to do. This is a special step in that it is engine-specific and has a syntax outside the normally permitted scope.

This is what I see in the snippet generator help text:

(No visual configuration.) Takes a map from branch names to closures 
and an optional argument failFast which will terminate all branches upon a failure in any other branch:

parallel firstBranch: {
    // do something
}, secondBranch: {
    // do something else
},
failFast: true|false

IMO it's not clear that the snippet generator will fail. If this is an accepted behaviour, I will note it clearly in the help text (as I'm currently working on JENKINS-30086).

No visual configuration.

is what I meant, but yes it could be better explained.

ParallelStep.DescriptorImpl could override newInstance(StaplerRequest, JSONObject) to return some sample ParallelStep with no branches, but this is not very helpful. Might be better for this to throw something (UnsupportedOperationException?) that would be rendered more politely in the result textarea, or even for StepDescriptor to be allowed to state that it has no visual configuration and to simply disable the snippet generation button in that case.

pipeline-syntax/html shows this error instead of documentation.