xfrm: update the stats documentation

Add a couple of stats that aren't in the documentation file
and rework the top description to be a little more readable.

Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Documentation/networking/xfrm_proc.txt

@@ -5,13 +5,15 @@ Masahide NAKAMURA <nakam@linux-ipv6.org>
 
 Transformation Statistics
 -------------------------
-xfrm_proc is a statistics shown factor dropped by transformation
-for developer.
-It is a counter designed from current transformation source code
-and defined like linux private MIB.
 
-Inbound statistics
-~~~~~~~~~~~~~~~~~~
+The xfrm_proc code is a set of statistics showing numbers of packets
+dropped by the transformation code and why.  These counters are defined
+as part of the linux private MIB.  These counters can be viewed in
+/proc/net/xfrm_stat.
+
+
+Inbound errors
+~~~~~~~~~~~~~~
 XfrmInError:
 	All errors which is not matched others
 XfrmInBufferError:
@@ -46,6 +48,10 @@ XfrmInPolBlock:
 	Policy discards
 XfrmInPolError:
 	Policy error
+XfrmAcquireError:
+	State hasn't been fully acquired before use
+XfrmFwdHdrError:
+	Forward routing of a packet is not allowed
 
 Outbound errors
 ~~~~~~~~~~~~~~~
@@ -72,3 +78,5 @@ XfrmOutPolDead:
 	Policy is dead
 XfrmOutPolError:
 	Policy error
+XfrmOutStateInvalid:
+	State is invalid, perhaps expired