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/Doc/c-api/unicode.rst b/Doc/c-api/unicode.rst
index a4ee03a..1b8402c 100644
--- a/Doc/c-api/unicode.rst
+++ b/Doc/c-api/unicode.rst
@@ -234,7 +234,7 @@
arguments, calculate the size of the resulting Python unicode string and return
a string with the values formatted into it. The variable arguments must be C
types and must correspond exactly to the format characters in the *format*
- string. The following format characters are allowed:
+ ASCII-encoded string. The following format characters are allowed:
.. % This should be exactly the same as the table in PyErr_Format.
.. % The descriptions for %zd and %zu are wrong, but the truth is complicated