xfs/040: document purpose

xfs/040: document purpose

Getting xfs/040 to "pass" takes a bit of effort, however the effort
to require updating xfsprogs is purely an xfsprogs maintainer task
only. There no functional gain by users of xfs or a QA team to get
this test to pass. This is not trivial from the current description
so document this.

Signed-off-by: Luis R. Rodriguez <mcgrof@kernel.org>
Reviewed-by: Bill O'Donnell <billodo@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>