gh-101100: Fix sphinx warnings in threading.rst

[sobolevn]

Original warnings:

/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:274: WARNING: py:mod reference target not found: _threading_local
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:283: WARNING: py:meth reference target not found: Thread.__init__
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:339: WARNING: py:class reference target not found: ThreadGroup
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:1012: WARNING: py:meth reference target not found: Timer.start
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:1150: WARNING: py:meth reference target not found: acquire
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:1150: WARNING: py:meth reference target not found: release
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:1150: WARNING: py:meth reference target not found: acquire
/Users/sobolev/Desktop/cpython/Doc/library/threading.rst:1150: WARNING: py:meth reference target not found: release

Couple of comments:

• _threading_local module is not documented as a module in sphinx
• Thread.__init__() is used the same way in other places as well, __init__ is not documented in sphinx
• ThreadGroup does not exist at the moment
• :meth:Timer.start <Thread.start> is used because Timer is Thread subclass and Timer.start is not documented
• release and acquire are used as general conventions, not exact methods

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

[hugovk]

Thanks!

[miss-islington]

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

[bedevere-bot]

GH-108707 is a backport of this pull request to the 3.12 branch.

[bedevere-bot]

GH-108708 is a backport of this pull request to the 3.11 branch.