dm thin: fix Documentation for held metadata root feature

The Documentation for the thin provisioning target's held metadata root
feature was incorrect.  It is now available and the value for the held
metadata root is in block units (not 512b sectors).
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

Documentation/device-mapper/thin-provisioning.txt

@@ -287,10 +287,9 @@ ii) Status
 	should register for the event and then check the target's status.
 
     held metadata root:
-	The location, in sectors, of the metadata root that has been
+	The location, in blocks, of the metadata root that has been
 	'held' for userspace read access.  '-' indicates there is no
-	held root.  This feature is not yet implemented so '-' is
-	always returned.
+	held root.
 
     discard_passdown|no_discard_passdown
 	Whether or not discards are actually being passed down to the