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: Neil Jerram <neil@tigera.io>
Signed-off-by: David S. Miller <davem@davemloft.net>