Mention the need to call `import_array();` if type depends on numpy

doc/extending/ctype.txt

@@ -94,6 +94,9 @@ the most important ones:
 
         Allows you to specify code that will be executed once when the
         module is initialized, before anything else is executed.
+        For instance, if a type depends on NumPy's C API, then
+        ``'import_array();'`` has to be among the snippets returned
+        by ``c_init_code()``.
 
     .. method:: c_support_code()