polish value_separator_index docstring (rfc role + Returns section)

[alhudz]

Linked issue

No issue to close. Follow-up to @stevepiercy's review on #1472 (the docstring landed just before the review).

Description

Applies the docstring tweaks @stevepiercy asked for on Contentline.value_separator_index:

• use the :rfc: cross-reference role for the RFC 5545 §3.1 reference instead of plain text, matching the rest of the codebase
• add a Returns section, per the docstring structure style guide

No behaviour change.

Checklist

• I've added a change log entry to /news, following the instructions in Change log entry format.
• I've added or updated tests if applicable. (not applicable, docstring only)
• I've run and ensured all tests pass locally by following Run tests.
• I've added or edited documentation, both as docstrings to be rendered in the API documentation and narrative documentation, as necessary.

Additional information

I used the #section-3.1 anchor to match the existing :rfc: usages in the repo.

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[read-the-docs-community]

Documentation build overview

📚 icalendar | 🛠️ Build #33265061 | 📁 Comparing e58b7a6 against latest (897bc51)

  🔍 Preview build  

4 files changed

± 404.html
± genindex.html
± reference/api/icalendar.parser.content_line.html
± _modules/icalendar/parser/content_line.html

[stevepiercy]

Don't forget credit!

Suggested change

	Add an RFC cross-reference and a ``Returns`` section to the ``Contentline.value_separator_index`` docstring.
	Add an RFC cross-reference and a ``Returns`` section to the ``Contentline.value_separator_index`` docstring. @alhudz

[stevepiercy]

Now that's a pretty docstring!

https://icalendar--1498.org.readthedocs.build/en/1498/reference/api/icalendar.parser.content_line.html#icalendar.parser.content_line.Contentline.value_separator_index