reworked documentation of JSONLDSerializer.

[WhiteGobo]

Still work in correspondence to #3307.
Added missing documentation arguments.
Working towards that all arguments are accesible with inspect.

Summary of changes

Added more documentation to module and JsonLDSerializer.serialize.
Moved Arguments to serialize arguments, instead of extracting them from **args.
No functional change, but because i dont extract anything from **args anymore, there are some changes in the program code.
Still missing explanation to argument context.
Added docstring to PLAIN_LITERAL_TYPES.
Added crosslink to graph.serialize

Checklist

• Checked that there aren't other open pull requests for
  the same change.
• Checked that all tests and type checking passes.
• If the change adds new features or changes the RDFLib public API:
  • Created an issue to discuss the change and get in-principle agreement.
  • Considered adding an example in ./examples.
• Considered granting push permissions to the PR branch,
  so maintainers can fix minor issues and keep your PR up to date.