[media] rc-map.h: document structs/enums on it

There are some structs/enums that aren't documented via
kernel-doc markup. Add documentation for them.

Fix those warnings:
./include/media/rc-map.h:103: WARNING: c:type reference target not found: rc_map_list
./include/media/rc-map.h:110: WARNING: c:type reference target not found: rc_map_list
./include/media/rc-map.h:117: WARNING: c:type reference target not found: rc_map

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>