[boyfeiyu]

The descriptor-lock guide currently describes both native backends as one-byte locks and recommends the API for conda-style byte-offset protocols. The implementation uses a whole-file flock on Unix and a one-byte LockFileEx range at offset 0 on Windows; seeking the descriptor does not select a different range.

Clarify that contract in the guide and API docstring, and replace the interoperability recommendation with the requirement to match the existing protocol's primitive and range. Conda's offset-21 protocol illustrates why the API is not a portable replacement. The Unix explanation accounts for operating-system and filesystem differences in flock/lockf interaction. Add a documentation changelog fragment.

Validation on macOS 27.2 arm64, Python 3.15.2:

• tox -e docs (Sphinx -W), with generated how-to, API and Unreleased content checked.
• Changed-file pre-commit checks and release-creator validate-news --base main.
• python -m pytest tests/test_filelock.py -k descriptor: 23 passed, 500 deselected.
• AST comparison confirms that executable code is unchanged; a cross-process probe checked local descriptor contention and byte-range controls.

Windows behavior was checked against the source and documented API contract, not executed. The local probe is not a conda integration test or a cross-platform test suite.