Issue #9738: PyUnicode_FromFormat() and PyErr_Format() raise an error on
a non-ASCII byte in the format string.

Document also the encoding.
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index 0de0d66..820850a 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -550,8 +550,14 @@
     register PyObject *obj      /* Object */
     );
 
-PyAPI_FUNC(PyObject *) PyUnicode_FromFormatV(const char*, va_list);
-PyAPI_FUNC(PyObject *) PyUnicode_FromFormat(const char*, ...);
+PyAPI_FUNC(PyObject *) PyUnicode_FromFormatV(
+    const char *format,   /* ASCII-encoded string  */
+    va_list vargs
+    );
+PyAPI_FUNC(PyObject *) PyUnicode_FromFormat(
+    const char *format,   /* ASCII-encoded string  */
+    ...
+    );
 
 /* Format the object based on the format_spec, as defined in PEP 3101
    (Advanced String Formatting). */