Documentation: filesystems: add new btrfs mount options

Two new options were added in 3.12: commit and rescan_uuid_tree

CC: linux-doc@vger.kernel.org
Signed-off-by: NDavid Sterba <dsterba@suse.cz>
Signed-off-by: NChris Mason <chris.mason@fusionio.com>

Documentation/filesystems/btrfs.txt

@@ -70,6 +70,12 @@ Unless otherwise specified, all options default to off.
 
 	See comments at the top of fs/btrfs/check-integrity.c for more info.
 
+  commit=<seconds>
+	Set the interval of periodic commit, 30 seconds by default. Higher
+	values defer data being synced to permanent storage with obvious
+	consequences when the system crashes. The upper bound is not forced,
+	but a warning is printed if it's more than 300 seconds (5 minutes).
+
   compress
   compress=<type>
   compress-force
@@ -154,6 +160,10 @@ Unless otherwise specified, all options default to off.
 	Currently this scans a list of several previous tree roots and tries to 
 	use the first readable.
 
+  rescan_uuid_tree
+	Force check and rebuild procedure of the UUID tree. This should not
+	normally be needed.
+
   skip_balance
 	Skip automatic resume of interrupted balance operation after mount.
 	May be resumed with "btrfs balance resume."