[3.6] bpo-25438: document what codec PyMemberDef T_STRING decodes the char * as ...

[3.6] bpo-25438: document what codec PyMemberDef T_STRING decodes the char * as (GH-10580) (GH-10587)

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

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

https://bugs.python.org/issue25438
(cherry picked from commit 689d555ec135d4115574addd063c358ac4897cc4)

Co-authored-by: Windson yang <wiwindson@outlook.com>
https://bugs.python.org/issue25438