gh-105912: document gotcha with using os.fork on macOS (#112871)

* gh-105912: document gotcha with using os.fork on macOS

Using ``fork(2)`` on macOS when also using higher-level
system APIs in the parent proces can crash on macOS because
those system APIs are not written to handle this usage
pattern.

There's nothing we can do about this other than documenting
the problem.

Co-authored-by: Carol Willing <carolcode@willingconsulting.com>