[3.15] gh-146219: Document reusing a thread state across repeated foreign-thread...

[3.15] gh-146219: Document reusing a thread state across repeated foreign-thread calls (GH-146221) (#152644)

gh-146219: Document reusing a thread state across repeated foreign-thread calls (GH-146221)

* Document reusing a thread state across repeated foreign-thread calls

Add a subsection under "Non-Python created threads" explaining the
performance cost of creating/destroying a PyThreadState on every
Ensure/Release cycle and showing how to keep one alive for the
thread's lifetime instead.

* add a comma

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(cherry picked from commit 5717518fb3ed6f34716683e08ec372267d871d21)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>
Co-authored-by: Peter Bierma <zintensitydev@gmail.com>