docs: improve pg_upgrade standby instructions

This makes it clear that pg_upgrade standby upgrade instructions should
only be used in link mode, adds examples, and explains how rsync works
with links.

Reported-by: Andreas Joseph Krogh

Discussion: https://postgr.es/m/VisenaEmail.6c.c0e592c5af4ef0a2.15e785dcb61@tc7-visena

Backpatch-through: 9.5