Fix note about type being different

Author: encukou

Doc/c-api/structures.rst

@@ -39,7 +39,8 @@ under :ref:`reference counting <countingrefs>`.
       Do not use this field directly; instead use functions and macros such as
       :c:macro:`!Py_REFCNT`, :c:func:`Py_INCREF` and :c:func:`Py_DecRef`.
 
-      The type may be different from `Py_ssize_t` on platforms.
+      The field type may be different from `Py_ssize_t`, depending on
+      build configuration and platform.
 
    .. c:member:: PyTypeObject* ob_type