[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>
diff --git a/Documentation/filesystems/xfs.txt b/Documentation/filesystems/xfs.txt
index 10ba81f..0a1668b 100644
--- a/Documentation/filesystems/xfs.txt
+++ b/Documentation/filesystems/xfs.txt
@@ -59,7 +59,8 @@
 	When ikeep is specified, XFS does not delete empty inode clusters
 	and keeps them around on disk. ikeep is the traditional XFS
 	behaviour. When noikeep is specified, empty inode clusters
-	are returned to the free space pool. The default is noikeep.
+	are returned to the free space pool. The default is noikeep for
+	non-DMAPI mounts, while ikeep is the default when DMAPI is in use.
 
   inode64
 	Indicates that XFS is allowed to create inodes at any location