docs: explain loop factory selection for parametrized asyncio tests

[lonh-jing]

In strict mode, adding an asyncio marker only through pytest.param does not trigger loop-factory parametrization. The parametrization guide currently does not mention this limitation.

Document where to place the marker, with an executable example showing two parameter values running against two loop factories. Also explain why mixed asyncio/Trio parameter sets need separately marked test functions. The example is collected by the existing documentation test suite.

Related to #1463. This documents the existing limitation and a working usage pattern; it does not resolve the underlying runtime issue.

Validation on macOS, Python 3.13.11, pytest 9.1.1:

• The new example passes all four cases with --asyncio-mode=strict.
• The parameter-specific marker reproduction fails to select the configured factories in strict mode; the same reproduction selects them in auto mode. Fixture parameter marks were checked separately.
• Sphinx builds with -n -W --keep-going; pre-commit checks pass for all changed files.
• Full suite: baseline 294 passed, 3 skipped, 1 failed; with this patch 298 passed, 3 skipped, 1 failed. The same existing failure in test_asyncio_run_after_async_fixture_does_not_leak_loop raises PytestAssertRewriteWarning about pytest_asyncio already being imported.

AI assistance: Codex (GPT-6 Astra) assisted with investigation, documentation, and executable examples. The local behavior checks above were actually run.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.50%. Comparing base (cb94811) to head (a9c03d0).

Additional details and impacted files

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  Hits          482      482           
  Misses         22       22           
  Partials        6        6           

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[tjkuson]

Thanks for your interest in improving the documentation!

I'm going to close this because the working usage is already covered by the existing documentation (which this PR somewhat duplicates), and the additional restriction pertains to an unresolved bug. I'd rather keep workaround attached to that issue rather than expand the guide around it. I'll also add that although #1463 is definitely a bug we should solve, the workaround is specific to code example posted by the reporter, and not a pattern that I'd like to bless in the documentation itself.