gh-46236: Add missing document of PyUnicode_DecodeCodePageStateful

`PyUnicode_DecodeCodePageStateful` is in the limited C-API,
and the document does not exist.

Author: Yuki Kobayashi

Doc/c-api/unicode.rst

@@ -1324,6 +1324,13 @@ the user settings on the machine running the codec.
    in *consumed*.
 
 
+.. c:function:: PyObject* PyUnicode_DecodeCodePageStateful(int code_page, const char *str, \
+                              Py_ssize_t size, const char *errors, Py_ssize_t *consumed)
+
+   Same as :c:func:`PyUnicode_DecodeMBCSStateful`, except that using the code page
+   specified by *code_page*.  Use :c:macro:`!CP_ACP` code page to get the MBCS decoder.
+
+
 .. c:function:: PyObject* PyUnicode_AsMBCSString(PyObject *unicode)
 
    Encode a Unicode object using MBCS and return the result as Python bytes

Doc/data/refcounts.dat

@@ -2636,6 +2636,13 @@ PyUnicode_DecodeMBCSStateful:Py_ssize_t:size::
 PyUnicode_DecodeMBCSStateful:const char*:errors::
 PyUnicode_DecodeMBCSStateful:Py_ssize_t*:consumed::
 
+PyUnicode_DecodeCodePageStateful:PyObject*::+1:
+PyUnicode_DecodeCodePageStateful:int:code_page::
+PyUnicode_DecodeCodePageStateful:const char*:s::
+PyUnicode_DecodeCodePageStateful:Py_ssize_t:size::
+PyUnicode_DecodeCodePageStateful:const char*:errors::
+PyUnicode_DecodeCodePageStateful:Py_ssize_t*:consumed::
+
 PyUnicode_EncodeCodePage:PyObject*::+1:
 PyUnicode_EncodeCodePage:int:code_page::
 PyUnicode_EncodeCodePage:PyObject*:unicode:0: