I'm trying to use
checkout gitSCM (some arguments here)

I see that one example, given at
https://www.jenkins.io/doc/pipeline/steps/params/scmgit/
is

checkout scmGit(userRemoteConfigs: [ [ url: 'https://github.com/jenkinsci/git-plugin' ] ])

Higher up in the page, it mentions,

" Refer to the git plugin documentation for detailed descriptions of options available to the checkout step"

But the referenced page is unusable for this sort of purpose.

I just want to find out what other things can be used in "userRemoteConfigs".

In the referenced page, I see various random examples of that used.
But nowhere do I see a concise reference of the approximate format,

"For userRemoteConfigs, here are the possible options and syntax.

  url: url to the remote repo
  xyz: ....
  abc: ....
"