staging/lustre: Add documentation for unstable_stats in sysfs

commit ac5b1481 ("staging: lustre: osc: Track and limit
"unstable" pages") added a new sysfs variable, but corresponding bit of
documentation was not forgotten.
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/staging/lustre/sysfs-fs-lustre

@@ -294,6 +294,14 @@ Description:
 		Controls extended attributes client-side cache.
 		1 to enable, 0 to disable.
 
+What:		/sys/fs/lustre/llite/<fsname>-<uuid>/unstable_stats
+Date:		Apr 2016
+Contact:	"Oleg Drokin" <oleg.drokin@intel.com>
+Description:
+		Shows number of pages that were sent and acknowledged by
+		server but were not yet committed and therefore still
+		pinned in client memory even though no longer dirty.
+
 What:		/sys/fs/lustre/ldlm/cancel_unused_locks_before_replay
 Date:		May 2015
 Contact:	"Oleg Drokin" <oleg.drokin@intel.com>