Add missing docstrings to _loop.py and _two_way_dict.py

[mastash3ff]

The utility modules src/textual/_loop.py and src/textual/_two_way_dict.py have several public functions and methods with incomplete or missing docstrings.

Specifically:

• _loop.py: loop_first, loop_last, and loop_first_last have only a terse one-line summary with no Args or Yields sections. The module also has no module-level docstring.
• _two_way_dict.py: TwoWayDict.__init__, __setitem__, __delitem__, __iter__, __len__, and __contains__ have no docstrings at all.

Happy to open a PR with these additions if this is welcome.

[github-actions]

We found the following entry in the FAQ which you may find helpful:

• Why do some key combinations never make it to my app?

Feel free to close this issue if you found an answer in the FAQ. Otherwise, please give us a little time to review.

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This is an automated reply, generated by FAQtory

[TomJGooding]

Closing as AI slop - see #6475 (comment)

[github-actions]

I hope I solved your issue!

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• Will McGugan

[willmcgugan]

I must write solid code if that is the only criticism an AI could come up with.