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/pyerrors.h b/Include/pyerrors.h
index 1eee16d..243bc01 100644
--- a/Include/pyerrors.h
+++ b/Include/pyerrors.h
@@ -183,7 +183,11 @@
     PyObject *, const Py_UNICODE *);
 #endif /* MS_WINDOWS */
 
-PyAPI_FUNC(PyObject *) PyErr_Format(PyObject *, const char *, ...);
+PyAPI_FUNC(PyObject *) PyErr_Format(
+    PyObject *exception,
+    const char *format,   /* ASCII-encoded string  */
+    ...
+    );
 
 #ifdef MS_WINDOWS
 PyAPI_FUNC(PyObject *) PyErr_SetFromWindowsErrWithFilenameObject(