ceph: fix description for rsize and rasize mount options

Signed-off-by: Andreas Gerstmayr <andreas.gerstmayr@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

Documentation/filesystems/ceph.txt

@@ -98,6 +98,10 @@ Mount Options
 	size.
 
   rsize=X
+	Specify the maximum read size in bytes.  By default there is no
+	maximum.
+
+  rasize=X
 	Specify the maximum readahead.
 
   mount_timeout=X