improve documentation about IP multicast group services to state implementation details

Some implementation details shall be given about IGMP membership handling in IP stack.

Especially information that there is no "per socket" storage done for joined multicast groups needs to be given. IP stack only counts "per IGMP group" how many sockets requested joining the group. As consequence, leaving a group may not automatically generate a leave message on the network. As consequence, a socket can successfully request leaving a group, even if the same socket did not join the group.