PyDoc: Use explicit class names to refence class

This fixes the link and makes it more clear which class the note is 
talking about

doc/python_api/rst/bgl.rst

@@ -21,8 +21,8 @@ offers a set of extensive examples, including advanced features.
 
 .. note::
 
-   You can use the :class:`Image` type to load and set textures.
-   See :class:`Image.gl_load` and :class:`Image.gl_free`,
+   You can use the :class:`bpy.types.Image` type to load and set textures.
+   See :class:`bpy.types.Image.gl_load` and :class:`bpy.types.Image.gl_free`,
    for example.