Add notes to pg_combinebackup docs

Hello,

While doing some work/research on the new incremental backup feature
some limitations were not listed in the docs. Mainly the fact that
pg_combienbackup works with plain format and not tar.

Around the same time, Tomas Vondra tested incremental backups with a
cluster where he enabled checksums after taking the previous full
backup. After combining the backups the synthetic backup had pages
with checksums and other pages without checksums which ended in
checksum errors.

I've attached two patches, the first one is just neat-picking things I
found when I first read the docs. The second has a note on the two
limitations listed above. The limitation on incremental backups of a
cluster that had checksums enabled after the previous backup, I was
not sure if that should go in pg_basebackup or pg_combienbackup
reference documentation. Or maybe somewhere else.

Kind regards, Martín

-- 
Martín Marqués
It’s not that I have something to hide,
it’s that I have nothing I want you to see