xfs: document dax mount option

index 8146e9f..e7e935d 100644
@@ -58,6 +58,11 @@ default behaviour.
 	This allows for drive level write caching to be enabled, for
 	devices that support write barriers.
 
+  dax
+	Use direct access (no page cache) for block devices which are
+	memory-like, performing reads and writes directly to the storage
+	device.  See Documentation/filesystems/dax.txt.
+
   discard
   nodiscard (*)
 	Enable/disable the issuing of commands to let the block