docs: add missing API docstrings

[w3lld1]

Summary

• Add docstrings for the remaining undocumented API objects listed in Some functions and properties in the documentation have no docstring #3221.
• Document ParkingLot.broken_by manually while excluding it from autodoc's undocumented-member warnings.
• Clear the now-empty undocumented allowlist and add a doc newsfragment.

Fixes #3221

Test plan

• black --check src/trio/_channel.py src/trio/_subprocess.py docs/source/conf.py
• python -m pytest src/trio/_tests/test_channel.py -q
• make -C docs html SPHINXOPTS='-W --keep-going'

[w3lld1]

Updated the type completeness allowlist after CI showed the newly documented channel classes and HasFileno.fileno entries are now resolved.

Local validation:

• python3 -m tox -m check --override testenv:gen_exports.base_python=python3 --override testenv:pip_compile.base_python=python3 --override testenv:type_completeness.base_python=python3

Note: local validation used python3 overrides because Python 3.13 is not installed in this runner; CI will still run the repository's normal Python 3.13 configuration.

[A5rocks]

This has already been partly solved by an existing PR