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.

Author: gpshead

Doc/howto/logging-cookbook.rst

@@ -1267,11 +1267,8 @@ to adapt in your own applications.
 
 You could also write your own handler which uses the :class:`~multiprocessing.Lock`
 class from the :mod:`multiprocessing` module to serialize access to the
-file from your processes. The existing :class:`FileHandler` and subclasses do
-not make use of :mod:`multiprocessing` at present, though they may do so in the
-future. Note that at present, the :mod:`multiprocessing` module does not provide
-working lock functionality on all platforms (see
-https://bugs.python.org/issue3770).
+file from your processes. The stdlib :class:`FileHandler` and subclasses do
+not make use of :mod:`multiprocessing`.
 
 .. currentmodule:: logging.handlers