pg_upgrade docs:  clarify rsync and move verification step
pg_upgrade docs:  clarify rsync and move verification step

These are adjustments based on someone using the new standby upgrade
steps.

Report by Andy Colson

Backpatch through 9.5

zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl,zeroae/postgres-xl