pyln_client: (re-)adds method description to usage via docstring

[m-schmoock]

The extended usage information (long_description) has been removed a while ago without proper replacement. ( commit 2ff3e55f0 Date: Mon Jul 29 17:58:43 2024 -0700 )

This PR does the following:

• adds a optional description parameter to a plugin method via docstring that will (if set) be appended with a newline following the generated usage argument list.
• adds method usage to plugin.print_usage()

• adds a testcase that checks if extended usage information is passed to manifest json and print_usage

  Things to think about

• Maybe we want to add an optional getmanifest json description field and use that instead of internally appending it with newline to the usage

[m-schmoock]

@cdecker Another option would be to reintroduce the description as a separate decorator to the method but do the same internal handling (appending to usage and passing to manifest json). Maybe that looks nicer in plugin code

[rustyrussell]

We had to fix it quickly for the release, BUT the correct answer is to use the docstring! We already introspect, so accessing doc is easy (which always exists, but is None for no doc string).

[m-schmoock]

We had to fix it quickly for the release, BUT the correct answer is to use the docstring! We already introspect, so accessing doc is easy (which always exists, but is None for no doc string).

Thanks for the clarification. I have been away from the project for a while as you know ;)

I rewrote the PR to use python docstring doc (if set) as the description.

[m-schmoock]

We may want to extend the JSON schema to have the description as a separate field (and not internally newline concatenated to the usage). What do you think?