Made the example more robust and also added a note for sharing entension types...

Made the example more robust and also added a note for sharing entension types as this is non-trivial.

docs/examples/userguide/extension_types/my_module.pyx

+from __future__ import print_function
+
+cdef class Shrubbery:
+
+    def __init__(self, w, h):
+        self.width = w
+        self.height = h
+
+    def describe(self):
+        print("This shrubbery is", self.width,
+              "by", self.height, "cubits.")

docs/src/userguide/extension_types.rst

@@ -115,6 +115,14 @@ The same consideration applies to local variables, for example:
 
 .. literalinclude:: ../../examples/userguide/extension_types/shrubbery_2.pyx
 
+.. note::
+
+    We here ``cimport`` the class :class:`Shrubbery`, and this is necessary
+    to declare the type at compile time. To be able to ``cimport`` an extension type,
+    we split the class definition into two parts, one in a definition file and
+    the other in the corresponding implementation file. You should read
+    :ref:`sharing_extension_types` to learn to do that.
+
 
 Type Testing and Casting
 ------------------------

docs/src/userguide/sharing_declarations.rst

@@ -203,6 +203,7 @@ example:
     this object from Python, nor can you use it from Cython using a normal import
     statement; you have to use :keyword:`cimport`.
 
+.. _sharing_extension_types:
 
 Sharing Extension Types
 =======================