doc/ip-sysctl: add bc_forwarding

Let's describe this sysctl.

Fixes: 5cbf777c ("route: add support for directed broadcast forwarding")
Signed-off-by: NNicolas Dichtel <nicolas.dichtel@6wind.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.rst

@@ -267,6 +267,13 @@ ipfrag_max_dist - INTEGER
 	from different IP datagrams, which could result in data corruption.
 	Default: 64
 
+bc_forwarding - INTEGER
+	bc_forwarding enables the feature described in rfc1812#section-5.3.5.2
+	and rfc2644. It allows the router to forward directed broadcast.
+	To enable this feature, the 'all' entry and the input interface entry
+	should be set to 1.
+	Default: 0
+
 INET peer storage
 =================