doc: added note about using the self.ids ObservablDict

doc/sources/guide/lang.rst

@@ -343,6 +343,10 @@ In your python code:
             self.ids.hulk.text = "hulk: puny god!"
             self.ids.loki.text = "loki: >_<!!!"
 
+When your kv file is parsed, kivy collects all the widgets tagged with id's
+and places them in this `self.ids` :class:`~kivy.properties.ObservableDict`
+property. That means you can iterate over these widgets and access them
+dictionary style.
 
 Dynamic Classes
 ---------------