docs: warn users that core image != widget image

doc/sources/guide/architecture.rst

@@ -14,6 +14,8 @@ Kivy consists of several building blocks that we will explain in the
 following.
 
 
+.. _providers:
+
 Core Providers and Input Providers
 ----------------------------------
 

kivy/core/__init__.py

@@ -4,13 +4,18 @@ Core Abstraction
 
 This module defines the abstraction layers for our core providers and their
 implementations. For further information, please refer to
-`Architectural Overview` and the `Core Providers and Input Providers` section
-of the documentation.
+:ref:`architecture` and the :ref:`providers` section of the documentation.
 
 In most cases, you shouldn't directly use a library that's already covered
 by the core abstraction. Always try to use our providers first.
 In case we are missing a feature or method, please let us know by
 opening a new Bug report instead of relying on your library.
+
+    **Note:**
+        These are **not** widgets! These are just abstractions of the respective
+        functionality. For example, you cannot add a core image to your window.
+        You have to use the image **widget** class instead. If you're really
+        looking for widgets, please refer to :mod:`kivy.uix` instead.
 '''
 
 import os