ANDROID: net: ip-sysctl: Document tcp_fwmark_accept

This documentation patch is the part of original tcp_fwmark_accept
implementation, Change-Id: I26bc1eceefd2c588d73b921865ab70e4645ade57
("net: support marking accepting TCP sockets").

The implementation since then got upstreamed, 84f39b08
("net: support marking accepting TCP sockets"), without this
documentation part.

Change-Id: I26bc1eceefd2c588d73b921865ab70e4645ade57
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>

[AmitP: Refactored the commit log.
        Fixes: 84f39b08 ("net: support marking accepting TCP sockets")
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>