design: Document project quota inode

design: Document project quota inode

A project quota inode was introduced into V5 superblock allowing
project quotas and group quotas to be used simultaneously. However
on "Quota Inodes" section we still mention it's not possible to
use project and group quotas at sametime.

This patch documents this behavior for V5 and later superblocks,
while keeps documented the old behavior for earlier versions.

Signed-off-by: Marco Benatto <mbenatto@redhat.com>
[darrick: rephrase slightly]
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Carlos Maiolino <cmaiolino@redhat.com>