docs: Mention that pg_combinebackup does not verify backups.

docs: Mention that pg_combinebackup does not verify backups.

We don't want users to think that pg_combinebackup is trying to check
the validity of individual backups, because it isn't. Adjust the wording
about sanity checks to make it clear that verification of individual
backups is the job of pg_verifybackup, and that the checks performed
by pg_combinebackup are around the relationships between the backups.

Per discussion with David Steele.

Discussion: http://postgr.es/m/e6f930c3-590c-47b9-b094-217bb2a3e22e@pgmasters.net