docs: document HasFileno.fileno and ParkingLot.broken_by (remaining #3221 items)

[bfs2021]

Resolves the two remaining checklist items of #3221 (trio._subprocess.HasFileno.fileno and trio.lowlevel.ParkingLot.broken_by). This also covers the remaining ground of #3406, whose memory-channel docstrings have already landed separately — the approach follows what was approved there.

What

• HasFileno.fileno: added a docstring describing the contract (return the file descriptor as an integer, which trio passes to the child process when a HasFileno object is used as stdio).
• ParkingLot.broken_by: documented via an Attributes: block in the class docstring (same pattern as MemoryChannelStatistics from docs: add docstrings to memory channel classes #3406), because the field is a slotted attrs attribute and autodoc reads its docstring at runtime, where a PEP-224-style attribute docstring would not be visible.
• Removed the now-satisfied trio._subprocess.HasFileno.fileno entry from _check_type_completeness.json (the docstring makes pyright stop reporting it), and removed trio.lowlevel.ParkingLot.broken_by from the UNDOCUMENTED set in docs/source/conf.py, since it now renders through the class docstring.
• Added :exclude-members: broken_by to the ParkingLot autoclass in reference-lowlevel.rst: with the Attributes: block plus :undoc-members:, the (still docstring-less at runtime) slotted field was rendered a second time as an empty entry, producing a duplicate-object-description warning.

Verification

• sphinx-build -n on docs/: zero warnings related to these members; both docstrings render (HasFileno.fileno in reference-io.html, the broken_by attributes block in reference-lowlevel.html).
• check_type_completeness.py runs cleanly against the updated JSON with respect to the touched symbols (the checker's output is pyright-version-dependent locally — the pre-existing baseline also reports unrelated drift — but HasFileno.fileno now resolves to a runtime docstring via inspect.getdoc, so the diagnostic is skipped exactly as the checker intends).
• ruff check / ruff format --check clean on touched files.

[A5rocks]

Duplicate of #3523