Add docstrings for HasFileno.fileno and ParkingLot.broken_by

[faresrafat3]

Fixes #3221.

trio._subprocess.HasFileno.fileno and ParkingLot.broken_by are both part
of the public interface but had no documentation. HasFileno.fileno was the
last remaining unchecked item in the "Some functions and properties in the
documentation have no docstring" checklist, and broken_by was rendered in
the low-level reference only because ParkingLot is documented with
:undoc-members:, so it showed up as a bare attribute name.

Changes

• HasFileno.fileno: added a docstring describing what the method returns and
  noting that it mirrors io.IOBase.fileno.
• ParkingLot: documented broken_by in the class's Attributes: section,
  matching how other Trio classes describe their attributes.
• _check_type_completeness.json: removed the now-resolved entry
  No docstring found for function "trio._subprocess.HasFileno.fileno".

Verification

The type completeness checker reports the entry as resolved rather than as a
new error:

$ python src/trio/_tests/check_type_completeness.py
********************
Checking Linux...
Congratulations, you have resolved existing errors! Please remove them from
.../_check_type_completeness.json
['No docstring found for function "trio._subprocess.HasFileno.fileno"']

After removing that entry, the checker passes clean on all three platforms
(Linux, Windows, Darwin).

pyright --verifytypes=trio still reports 99.6% completeness, with the
missing-docstring counts unchanged for every other symbol.

The parking-lot tests pass:

$ pytest src/trio/_core/_tests/test_parking_lot.py
11 passed

The failures in test_exports.py::test_static_tool_sees_all_symbols are
pre-existing in this environment (they require pylint and jedi, which are
not installed here) and are unaffected by this change; I confirmed they fail
identically on the unmodified checkout.

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[A5rocks]

I prefer #3523