Document packages_distributions metadata fallback

[aryansk]

Summary

• Document how packages_distributions() uses the legacy top_level.txt metadata when present and falls back to installed-file inference.
• Explain why build backends are not required to provide top_level.txt and why mappings can be incomplete.
• Add the required release news fragment.

Fixes #526

Testing

• uv run --project . --python /opt/homebrew/bin/python3.14 --extra test pytest -q tests/test_api.py — 37 passed, 39 subtests passed
• uv run --project . --extra doc python -m sphinx -W --keep-going docs /tmp/codex-oss-next5.2ZwarK/importlib-docs-build — passed
• git diff --check