[3.11] [3.12] gh-109649: Enhance os.cpu_count() documentation (GH-110169) (#110226)

[3.11] [3.12] gh-109649: Enhance os.cpu_count() documentation (GH-110169) (#110226)

[3.12] gh-109649: Enhance os.cpu_count() documentation (GH-110169)

* gh-109649: Enhance os.cpu_count() documentation

* Doc: Specify that os.cpu_count() counts *logicial* CPUs.
* Doc: Specify that os.sched_getaffinity(0) is related to the calling
  thread.
* Fix test_posix.test_sched_getaffinity(): restore the old CPU mask
  when the test completes!

* Restore removed text
(cherry picked from commit 5245b97e132ae071e2b574224e0788cab62fdcc9)

Co-authored-by: Victor Stinner <vstinner@python.org>