Document tasks recorded in ParkingLot.broken_by

[GreedyC]

Document the remaining ParkingLot.broken_by item from #3221: the initially empty list, call order, implicit current task, and repeated entries when a lot is broken more than once.

The attribute is documented in the class's Google-style Attributes section and excluded from automatic member enumeration to avoid a duplicate empty entry. Its missing-docstring exemption is removed. The HasFileno.fileno item remains separate and is already covered by #3523; this PR does not close the tracking issue.

Validation:

• pytest src/trio/_core/_tests/test_parking_lot.py -q: 11 passed. Existing tests cover repeated and explicit breaker tasks.
• sphinx-build -b html -W --keep-going docs/source docs/build/html: passed.
• Checked the generated HTML contains exactly one ParkingLot.broken_by anchor with the explanation.
• Ruff lint/format and git diff --check passed.

Includes a documentation newsfragment. Runtime behavior is unchanged.

AI assistance: Codex helped prepare the documentation and perform the checks.