pg_combinebackup: Detect checksum mismatches and document limitation.

pg_combinebackup: Detect checksum mismatches and document limitation.

If not all backups have the same checksum status, but the final backup
has checksums enabled, then the output directory may include pages
with invalid checksums. Document this limitation and explain how to
work around it.

In a future release, we may want to teach pg_combinebackup to
recompute page checksums when required, but as feature freeze has come
and gone, it seems a bit too late to do that for this release.

Patch by me, reviewed by Daniel Gustafsson

Discussion: http://postgr.es/m/CA+TgmoZugzOSmgkx97u3pc0M7U8LycWvugqoyWBv6j15a4hE5g@mail.gmail.com