ocfs2: update mount option documentation

We forgot to document the atime_quantum mount option in ocfs2.txt. This adds
a proper description of how it works.
Signed-off-by: Tiger Yang <tiger.yang@oracle.com>
Signed-off-by: Mark Fasheh <mark.fasheh@oracle.com>

Documentation/filesystems/ocfs2.txt

@@ -54,3 +54,6 @@ errors=panic		Panic and halt the machine if an error occurs.
 intr		(*)	Allow signals to interrupt cluster operations.
 nointr			Do not allow signals to interrupt cluster
 			operations.
+atime_quantum=60(*)	OCFS2 will not update atime unless this number
+			of seconds has passed since the last update.
+			Set to zero to always update atime.