Add doc for `c_element_type()` into tutorial for Theano types in C.

doc/extending/ctype.txt

@@ -130,6 +130,16 @@ the most important ones:
        change in the code. If you don't want to cache the compiled code
        return an empty tuple or don't implement it.
 
+    .. method:: c_element_type()
+
+       Optional: should return the name of the primitive C type of
+       items into variables handled by this Theano type. For example,
+       for a matrix of 32-bit signed NumPy integers, it should return
+       ``"npy_int32"``. If C type may change from an instance to another
+       (e.g. ``Scalar('int32')`` vs ``Scalar('int64')``), consider
+       implementing this method. If C type is fixed accross instances,
+       this method may be useless (as you already know the C type
+       when you work with the C code).
 
 Each of these functions take two arguments, ``name`` and ``sub`` which
 must be used to parameterize the C code they return. ``name`` is a