Update gid API documentation. (#274)
Update gid API documentation. (#274)

Namely, rmw_get_gid_for_publisher() and rmw_compare_gids_equal().

Signed-off-by: Michel Hidalgo <2c7f9fd20fbeb41ce8894ec4653d66fa7f3b6e1a@ekumenlabs.com>
ros2/rmw,ros2/rmw