xfsdocs: capture some information about dirs vs. attrs and how they use dabtrees

xfsdocs: capture some information about dirs vs. attrs and how they use dabtrees

Dave and I had a short discussion about whether or not xattr trees
needed to have the same free space tracking that directories have, and
a comparison of how each of the two metadata types interact with
dabtrees resulted.  I've reworked this a bit to make it flow better as a
book chapter, so here we go.

Original-mail: https://lore.kernel.org/linux-xfs/20200404085203.1908-1-chandanrlinux@gmail.com/T/#mdd12ad06cf5d635772cc38946fc5b22e349e136f
Originally-from: Dave Chinner <david@fromorbit.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Reviewed-by: Brian Foster <bfoster@redhat.com>