[IPV4]: Document icmp_errors_use_inbound_ifaddr sysctl

Taken largely from the commit of the patch that added this feature:

1c2fb7f

I'm not sure about the ordering of the options in sysctl.txt,
so I took a wild guess about where it fits.
    
Signed-Off-By: Horms <horms@verge.net.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -427,6 +427,23 @@ icmp_ignore_bogus_error_responses - BOOLEAN
 	will avoid log file clutter.
 	Default: FALSE
 
+icmp_errors_use_inbound_ifaddr - BOOLEAN
+
+	If zero, icmp error messages are sent with the primary address of
+	the exiting interface.
+
+	If non-zero, the message will be sent with the primary address of
+	the interface that received the packet that caused the icmp error.
+	This is the behaviour network many administrators will expect from
+	a router. And it can make debugging complicated network layouts
+	much easier. 
+
+	Note that if no primary address exists for the interface selected,
+	then the primary address of the first non-loopback interface that
+	has one will be used regarldess of this setting.
+
+	Default: 0
+
 igmp_max_memberships - INTEGER
 	Change the maximum number of multicast groups we can subscribe to.
 	Default: 20