[3.13] gh-142302: Fix mkstemp() documentation: clarify file descriptor inheritance behavior (GH-142338) (#143487)

gh-142302: Fix mkstemp() documentation: clarify file descriptor inheritance behavior (GH-142338)

The documentation incorrectly stated that the file descriptor is not
inherited by child processes. In reality, the close-on-exec flag (when
available) only prevents inheritance across exec() calls, not fork().
(cherry picked from commit e79c9b703117d8b0e7c4d86b704e2e7f120db883)

Co-authored-by: ADITYA RAI <adi.hack1234@gmail.com>
Co-authored-by: Victor Stinner <vstinner@python.org>