Mention that "Latest checkpoint location" will not match in pg_upgrade
if the standby server is still running during the upgrade, which is
possible.  "Match" text first appeared in PG 9.5.

Reported-by: Paul Bonaud

Discussion: https://postgr.es/m/c7268794-edb4-1772-3bfd-04c54585c24e@trainline.com

Backpatch-through: 9.5