You're correct that the "add additional repositories" option is a source of confusion. Other users assumed it was some way to include multiple independent repositories in a single workspace. Rather, it handles only the case that you're describing, additional additional remotes as sources for the same repository.
The Pipeline Syntax hint for the checkout step presents a similar user interface as is available with Freestyle jobs. It is quite a helpful utility to construct Pipeline syntax. It is available as:
It is not a widely used or well-known feature because it has significant limitations on its use. I don't especially want to document its use because it is such an advanced feature. You're welcome to use it. There are several automated tests which confirm that it is still working, but it is a narrow enough use that I'd rather not publicize it widely.
If you feel strongly that it should be documented, you're welcome to submit documentation to the git plugin wiki page or to submit a pull request to jenkins.io with documentation.
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