Fix groupchat event related variable

The from child follow this pattern: <room_name>@conference.<host_name>/<user_nick>

Signed-off-by: Emilien Kenler <hello@emilienkenler.com>
Add some documentation

Signed-off-by: Emilien Kenler <hello@emilienkenler.com>
Add one groupchat example

Signed-off-by: Emilien Kenler <hello@emilienkenler.com>