[3.13] Document that multiprocessing treats local same-user processes as trusted (GH-149001) (#149034)

Document that multiprocessing treats local same-user processes as trusted (GH-149001)

Clarify in the Authentication keys section that the authkey handshake
covers Listener/Client (addressable endpoints) only, not the anonymous
pipes behind Pipe() and Queue, and that isolation between same-user
processes must be arranged at the OS level.
(cherry picked from commit f27e91e37212f148b8fe72a3656a69b242625622)

Co-authored-by: Gregory P. Smith <68491+gpshead@users.noreply.github.com>