Document subprocess worker-thread context inheritance

[roli-lpci]

Summary

• document that trio.run_process and trio.lowlevel.open_process call
  subprocess.Popen from a worker thread
• explain that applicable per-thread operating-system state is therefore
  inherited from the worker thread, with Linux namespaces and CPU affinity as
  examples
• add the corresponding documentation release note

This addresses the documentation request in #3360. It leaves the API and
spawning behavior unchanged; #3462's synchronous-spawn option was closed because
blocking the Trio event loop is unacceptable.

Validation

• a focused runtime probe wrapped the real subprocess.Popen, launched a child,
  and confirmed that Popen ran on a different OS thread from Trio's event loop
• sphinx-build -W --keep-going -b html docs/source ...
• towncrier build --draft
• file-scoped pre-commit checks for the two changed files

[A5rocks]

I'm not willing to waste my time reviewing this given how many open PRs you have + LLM generated PR description. I also think it's not exactly right because I don't think docs really address the issue, or at least I'm not yet convinced there isn't a solution.