ip: update the description of rp_filter in ip-sysctl.txt

The commit 27fed417 (ip: fix logic of
reverse path filter sysctl) has changed the logic of rp_filter.  The
document about rp_filter is out of date. Now, setting
conf/all/rp_filte with 0 can also enable source validation.

Update the document according to the commit.
Signed-off-by: NShan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

Documentation/networking/ip-sysctl.txt

@@ -738,8 +738,8 @@ rp_filter - INTEGER
 	to prevent IP spoofing from DDos attacks. If using asymmetric routing
 	or other complicated routing, then loose mode is recommended.
 
-	conf/all/rp_filter must also be set to non-zero to do source validation
-	on the interface
+	The max value from conf/{all,interface}/rp_filter is used
+	when doing source validation on the {interface}.
 
 	Default value is 0. Note that some distributions enable it
 	in startup scripts.