bpo-40421: Fix PyFrame_GetCode() documentation (GH-31535)

Fix PyFrame_GetBack() and PyFrame_GetCode() return type in
the documentation.
(cherry picked from commit 78859e5)

Co-authored-by: Victor Stinner <vstinner@python.org>

Doc/c-api/reflection.rst

@@ -31,7 +31,7 @@ Reflection
    See also :c:func:`PyThreadState_GetFrame`.
 
 
-.. c:function:: int PyFrame_GetBack(PyFrameObject *frame)
+.. c:function:: PyFrameObject* PyFrame_GetBack(PyFrameObject *frame)
 
    Get the *frame* next outer frame.
 
@@ -42,7 +42,7 @@ Reflection
    .. versionadded:: 3.9
 
 
-.. c:function:: int PyFrame_GetCode(PyFrameObject *frame)
+.. c:function:: PyCodeObject* PyFrame_GetCode(PyFrameObject *frame)
 
    Get the *frame* code.