Doc: Refer to ip-sysctl.txt for strict vs. loose rp_filter mode

The IP_ADVANCED_ROUTER Kconfig describes the rp_filter
proc option.  Recent changes added a loose mode.
Instead of documenting this change too places, refer to
the document describing it:
 Documentation/networking/ip-sysctl.txt

I'm considering moving the rp_filter description away
from the Kconfig file into ip-sysctl.txt.
Signed-off-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/Kconfig

@@ -50,6 +50,8 @@ config IP_ADVANCED_ROUTER
 	  echo 1 > /proc/sys/net/ipv4/conf/all/rp_filter
 
 	  Note that some distributions enable it in startup scripts.
+	  For details about rp_filter strict and loose mode read
+	  <file:Documentation/networking/ip-sysctl.txt>.
 
 	  If unsure, say N here.