docs: Fix pg_upgrade standby server upgrade docs

docs:  Fix pg_upgrade standby server upgrade docs

It was unsafe to instruct users to start/stop the server after
pg_upgrade was run but before the standby servers were rsync'ed.  The
new instructions avoid this.

RELEASE NOTES:  This fix should be mentioned in the minor release notes.

Reported-by: Dmitriy Sarafannikov and Sergey Burladyan

Discussion: https://postgr.es/m/87wp8o506b.fsf@seb.koffice.internal

Backpatch-through: 9.5, where standby server upgrade instructions first appeared