Added extension types mention for typing.

docs/src/userguide/language_basics.rst

@@ -83,10 +83,20 @@ Note that Cython uses array access for pointer dereferencing, as ``*x`` is not v
 whereas ``x[0]`` is.
 
 Also, the Python types ``list``, ``dict``, ``tuple``, etc. may be used for
-static typing, as well as any user defined extension types.  The Python types
-int, long, and float are not available for static typing and instead interpreted as C
-``int``, ``long``, and ``float`` respectively, as statically typing variables with these Python
+static typing, as well as any user defined :ref:`extension-types`.
+For example::
+
+    cdef list foo = []
+
+This requires an *exact* match of the class, it does not allow
+subclasses. This allows Cython to optimize code by accessing
+internals of the builtin class.
+For this kind of typing, Cython uses internally a C variable of type ``PyObject*``.
+The Python types int, long, and float are not available for static
+typing and instead interpreted as C ``int``, ``long``, and ``float``
+respectively, as statically typing variables with these Python
 types has zero advantages.
+
 While these C types can be vastly faster, they have C semantics.
 Specifically, the integer types overflow
 and the C ``float`` type only has 32 bits of precision
@@ -95,6 +105,12 @@ and is typically what one wants).
 If you want to use these numeric Python types simply omit the
 type declaration and let them be objects.
 
+It is also possible to declare :ref:`extension-types` (declared with ``cdef class``).
+This does allow subclasses. This typing is mostly used to access
+``cdef`` methods and attributes of the extension type.
+The C code uses a variable which is a pointer to a structure of the
+specific type, something like ``struct MyExtensionTypeObject*``.
+
 
 Grouping multiple C declarations
 --------------------------------