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document about rp_filter is out of date.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: Shan Wei <shanwei@cn.fujitsu.com>
Signed-off-by: David S. Miller <davem@davemloft.net>