Added useful doc on CollisionElement::collision_groups_
Added useful doc on CollisionElement::collision_groups_

sheim/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,sheim/drake,billhoffman/drake,billhoffman/drake,billhoffman/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,sheim/drake,billhoffman/drake,billhoffman/drake