widget: enhance the documentation of Widget to include more information / disclamer about the defaults of the Widget

kivy/uix/widget.py

@@ -33,6 +33,24 @@ Our widget class is designed with a couple of principles in mind:
         You can also check if a widget collides with another widget with
         :meth:`Widget.collide_widget`.
 
+
+We also have some defaults that you should be aware of:
+
+* A :class:`Widget` is not a :class:`Layout`: it will not change the position
+  nor the size of its children. If you want a better positionning / sizing, use
+  a :class:`Layout`.
+
+* The default size is (100, 100), if the parent is not a :class:`Layout`. For
+  example, adding a widget inside a :class:`Button`, :class:`Label`, will not
+  inherit from the parent size or pos.
+
+* The default size_hint is (1, 1). If the parent is a :class:`Layout`, then the
+  widget size will be the parent/layout size.
+
+* All the :meth:`Widget.on_touch_down`, :meth:`Widget.on_touch_move`,
+  :meth:`Widget.on_touch_up` doesn't do any sort of collisions. If you want to
+  know if the touch is inside your widget, use :meth:`Widget.collide_point`.
+
 Using Properties
 ----------------
 
@@ -54,6 +72,8 @@ widget moves, you can bind your own callback function like this::
     wid = Widget()
     wid.bind(pos=callback_pos)
 
+Read more about the :doc:`/api-kivy.properties`.
+
 '''
 
 __all__ = ('Widget', 'WidgetException')