Make IP 'forwarding' doc more precise

It wasn't clear if the 'forwarding' setting needs to be enabled on the
interface that packets are received from, or on the interface that
packets are forwarded to, or both.

In fact (according to my code reading) the setting is relevant on the
interface that packets are received from, so this change updates the doc
to say that.
Signed-off-by: NNeil Jerram <neil@tigera.io>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -1006,7 +1006,8 @@ accept_redirects - BOOLEAN
 		FALSE (router)
 
 forwarding - BOOLEAN
-	Enable IP forwarding on this interface.
+	Enable IP forwarding on this interface.  This controls whether packets
+	received _on_ this interface can be forwarded.
 
 mc_forwarding - BOOLEAN
 	Do multicast routing. The kernel needs to be compiled with CONFIG_MROUTE