docs: document multiprocessing fork warning workaround for #1186

[golikovichev]

This adds a short docs section explaining the multiprocessing fork warning that surfaces in #1186 and pointing readers at the spawn-context workaround that @RonnyPfannschmidt has suggested in the thread.

The intent is to make the workaround discoverable from the project docs rather than only from the issue comments, while the upstream execnet#336 experiment is still in progress. Once that lands and the warning stops appearing for the fork case, this section can either be trimmed to a note about historical 3.x behaviour or removed entirely.

Closes nothing. The issue should stay open until the execnet-side fix lands.

Links:

• Issue: pytest-xdist causes warnings to be emitted when a unit test uses os.fork() #1186
• Upstream fix in flight: use regular Thread objects for thread execnet model execnet#336

The new section is added at the end of docs/known-limitations.rst since that page already lists xdist quirks with workarounds. If you would prefer it under a different page (e.g. a future FAQ), let me know and I will move it.