elm_widget: added internal documentation about elm_widget_sub_object_add() and elm_widget_sub_object_parent_add().
elm_widget: added internal documentation about elm_widget_sub_object_add() and elm_widget_sub_object_parent_add().

rvandegrift/elementary,tasn/elementary,FlorentRevest/Elementary,FlorentRevest/Elementary,rvandegrift/elementary,FlorentRevest/Elementary,rvandegrift/elementary,tasn/elementary,tasn/elementary,tasn/elementary,rvandegrift/elementary,FlorentRevest/Elementary,tasn/elementary