gh-101100: Fix Sphinx warnings in library/calendar.rst

[hugovk]

Fix these five warnings:

• the first by documenting it
• the next two each refer to methods of two subclasses, so it's not obvious which subclass (or superclass)'s methods to link to, so just silence with !
• the last two by including the relevant class in the link

Doc/library/calendar.rst:232: WARNING: py:meth reference target not found: formatmonthname Doc/library/calendar.rst:292: WARNING: py:meth reference target not found: formatweekday Doc/library/calendar.rst:292: WARNING: py:meth reference target not found: formatmonthname Doc/library/calendar.rst:361: WARNING: py:meth reference target not found: formatmonth Doc/library/calendar.rst:372: WARNING: py:meth reference target not found: formatyear 

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--113500.org.readthedocs.build/

[serhiy-storchaka]

LGTM.

[miss-islington-app]

Thanks @hugovk for the PR 🌮🎉.. I'm working now to backport this PR to: 3.11, 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @hugovk, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker fb02b6696002fc3dedbcf7c58aa40301650936ba 3.12 

[bedevere-app]

GH-113508 is a backport of this pull request to the 3.11 branch.

[bedevere-app]

GH-113509 is a backport of this pull request to the 3.12 branch.