net: Document xfrm4_gc_thresh and xfrm6_gc_thresh

This change adds documentation for xfrm4_gc_thresh and xfrm6_gc_thresh
based on the comments in commit eeb1b73378b56 ("xfrm: Increase the garbage
collector threshold").

Signed-off-by: Alexander Duyck <alexander.h.duyck@redhat.com>
Signed-off-by: Steffen Klassert <steffen.klassert@secunet.com>

Documentation/networking/ip-sysctl.txt

@@ -1181,6 +1181,11 @@ tag - INTEGER
 	Allows you to write a number, which can be used as required.
 	Default value is 0.
 
+xfrm4_gc_thresh - INTEGER
+	The threshold at which we will start garbage collecting for IPv4
+	destination cache entries.  At twice this value the system will
+	refuse new allocations.
+
 Alexey Kuznetsov.
 kuznet@ms2.inr.ac.ru
 
@@ -1617,6 +1622,11 @@ ratelimit - INTEGER
 	otherwise the minimal space between responses in milliseconds.
 	Default: 1000
 
+xfrm6_gc_thresh - INTEGER
+	The threshold at which we will start garbage collecting for IPv6
+	destination cache entries.  At twice this value the system will
+	refuse new allocations.
+
 
 IPv6 Update by:
 Pekka Savola <pekkas@netcore.fi>