XFS: Update MAINTAINERS to catch all XFS documentation

Assumes that all XFS documentation will be prefixed with xfs-, which
seems like a good policy anyway.

Reviewed-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>