Document ParkingLot.broken_by

[bpiper02]

Closes Part of #3221

Adds documentation for ParkingLot.broken_by and removes it from the documentation allowlist.

Validation:

• python -m pytest src\trio\_core\_tests\test_parking_lot.py
• python -m ruff check src\trio\_core\_parking_lot.py docs\source\conf.py
• python -m ruff format --check src\trio\_core\_parking_lot.py docs\source\conf.py
• python -m compileall src\trio\_core\_parking_lot.py docs\source\conf.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (a3d7cdd) to head (c98feb8).

Additional details and impacted files

@@               Coverage Diff               @@
##                 main        #3508   +/-   ##
===============================================
  Coverage   100.00000%   100.00000%           
===============================================
  Files             128          128           
  Lines           19454        19455    +1     
  Branches         1321         1321           
===============================================
+ Hits            19454        19455    +1     

Files with missing lines	Coverage Δ
src/trio/_core/_parking_lot.py	100.00000% <100.00000%> (ø)

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[A5rocks]

As long as you don't try to use an LLM to do it, probably.

That applies to PR description too 😭