bpo-43293: Doc: move note about GIL to top of threading module (GH-24622)

bpo-43293: Doc: move note about GIL to top of threading module (GH-24622)

The note about the GIL was buried pretty deep in the threading documentation,
and this made it hard for first time users to discover why their attempts
at using threading to parallelizing their application did not work.

In this commit, the note is moved to the top of the module documention for
visibility.
(cherry picked from commit 32181be6081f6c70a1e0bd0540050805c8e88e83)

Co-authored-by: Guanzhong Chen <quantum2048@gmail.com>