[IPV4] ARP: Documentation for new arp_accept sysctl variable.

As John pointed out, I had not added documentation to describe the
arp_accpet sysctl that I posted in my last patch.  This patch adds
that documentation.
Signed-off-by: NNeil Horman <nhorman@tuxdriver.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -619,6 +619,11 @@ arp_ignore - INTEGER
 	The max value from conf/{all,interface}/arp_ignore is used
 	when ARP request is received on the {interface}
 
+arp_accept - BOOLEAN
+	Define behavior when gratuitous arp replies are received:
+	0 - drop gratuitous arp frames
+	1 - accept gratuitous arp frames
+
 app_solicit - INTEGER
 	The maximum number of probes to send to the user space ARP daemon
 	via netlink before dropping back to multicast probes (see