[XFS] Update XFS documentation for noikeep/ikeep.

Mention how DMAPI affects default for noikeep.
Slightly modified since Josef's patch was based on
an old xfs.txt prior to Dave's (dgc) checkin which
missed going to oss.

Signed-off-by: Josef Sipek <jeffpc@josefsipek.net>
Signed-off-by: Tim Shimmin <tes@sgi.com>