Docs: explain parametrization requirements with pytest-xdist

[LaTranquillum]

Parallel workers can collect different test IDs or collect them in a different order when parameters come from unordered inputs or values generated during collection. The parametrization how-to now explains the resulting collection error, shows a sorted-parameter example, and distinguishes stable IDs from consistent test cases. It links to pytest-xdist’s collection guidance and includes a changelog entry.

Closes #4101.

Validation on macOS:

• Reproduced the collection mismatch by reversing parameter order in one of two workers.
• Sorting those inputs passed both parametrized cases; the exact documented example also passed both cases (pytest 9.1.1, pytest-xdist 3.8.0, CPython 3.14.2).
• Full Sphinx HTML build passed with -W --keep-going after correcting a heading underline.
• blacken-docs --check and codespell passed for both changed files.

No runtime code changed; the full pytest test suite and full pre-commit suite were not run. Maintainer edits are enabled.

AI assistance: Codex drafted the patch and ran the checks. I reviewed the patch and explanation and authorized this submission.