Documentation: ip-sysctl.txt: document addr_gen_mode

addr_gen_mode was introduced in without documentation, add it now.

Fixes: d35a00b8

 ("net/ipv6: allow sysctl to change link-local address generation mode")
Signed-off-by: Sabrina Dubroca <sd@queasysnail.net>
Reviewed-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>