document how to clear edge group

[lolopinto]

right now, it's custom action and call clearEdgeTypeInGroup

https://github.com/lolopinto/ent/blob/main/examples/simple/src/schema/event_schema.ts#L123-L127
https://github.com/lolopinto/ent/blob/main/examples/simple/src/ent/event/actions/clear_event_rsvp_status_action.ts#L15-L26

#1202 which added it is relevant