docs: tproxy: ignore non-transparent sockets in iptables

The iptables example was added in commit d2f26037a38a (netfilter: Add
documentation for tproxy, 2008-10-08), but xt_socket 'transparent'
option was added in commit a31e1ffd2231 (netfilter: xt_socket: added new
revision of the 'socket' match supporting flags, 2009-06-09).

Now add the 'transparent' option to the iptables example to ignore
non-transparent sockets, which is also consistent with the nft example.

Signed-off-by: 谢致邦 (XIE Zhibang) <[e-mail]>
Signed-off-by: Pablo Neira Ayuso <[e-mail]>

Documentation/networking/tproxy.rst

···
 socket on your box, set the packet mark to a certain value::
 
     # iptables -t mangle -N DIVERT
-    # iptables -t mangle -A PREROUTING -p tcp -m socket -j DIVERT
+    # iptables -t mangle -A PREROUTING -p tcp -m socket --transparent -j DIVERT
     # iptables -t mangle -A DIVERT -j MARK --set-mark 1
     # iptables -t mangle -A DIVERT -j ACCEPT