[3.13] gh-48020: [docs] Remove the logging howto suggested future FileHandler multiprocessing support (GH-126531) (#126540)

gh-48020: [docs] Remove the logging howto suggested future FileHandler multiprocessing support (GH-126531)

Docs: Remove the logging howto potential promise of multiprocessing support in the future.

Stick to the facts and suggestions, don't provide hope where we're not going to
implement complexity that we'd rather the user implement themselves when
needed.
(cherry picked from commit 1fe67df8e373a5177143e4a310c83438e79f9b77)

Co-authored-by: Gregory P. Smith <greg@krypto.org>