Documentation: networking: ip-sysctl: Document src_valid_mark sysctl

Provide documentation for src_valid_mark sysctl, which was added
in commit 28f6aeea ("net: restore ip source validation").

Signed-off-by: Jay Vosburgh <jay.vosburgh@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>