Clarify cdef vs. def function arguments.

docs/src/userguide/language_basics.rst

@@ -117,7 +117,14 @@ using normal C declaration syntax. For example,::
 
 When a parameter of a Python function is declared to have a C data type, it is
 passed in as a Python object and automatically converted to a C value, if
-possible. Automatic conversion is currently only possible for numeric types,
+possible. In other words, the definition of `spam` above is equivalent to writing::
+
+    def spam(python_i, python_s):
+        int i = python_i
+        char* s = python_s
+        ...
+
+Automatic conversion is currently only possible for numeric types,
 string types and structs (composed recusively of any of these types);
 attempting to use any other type for the parameter of a
 Python function will result in a compile-time error.