bpo-25438: document what codec PyMemberDef T_STRING decodes the char * as (GH-10580)

Source of T_STRING: https://github.com/python/cpython/blob/e42b705188271da108de42b55d9344642170aa2b/Python/structmember.c#L51

Source of PyUnicode_FromString
https://github.com/python/cpython/blob/master/Include/unicodeobject.h#L702


https://bugs.python.org/issue25438

Doc/c-api/structures.rst

@@ -292,7 +292,8 @@ definition with the same method name.
 
    :attr:`flags` can be ``0`` for write and read access or :c:macro:`READONLY` for
    read-only access.  Using :c:macro:`T_STRING` for :attr:`type` implies
-   :c:macro:`READONLY`.  Only :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX`
+   :c:macro:`READONLY`.  :c:macro:`T_STRING` data is interpreted as UTF-8.
+   Only :c:macro:`T_OBJECT` and :c:macro:`T_OBJECT_EX`
    members can be deleted.  (They are set to *NULL*).