Update Doc/c-api/unicode.rst

Co-authored-by: Victor Stinner <[email protected]>

Author: StanFromIreland

Doc/c-api/unicode.rst

@@ -650,8 +650,8 @@ APIs:
    Return a mapping suitable for decoding a custom single-byte encoding.
    Given a Unicode string *string* of up to 256 characters representing an encoding
    table, returns either a compact internal mapping object or a dictionary
-   mapping character ordinals to byte values. Raises a :exc:`TypeError` on
-   invalid input.
+   mapping character ordinals to byte values. Raises a :exc:`TypeError` and
+   return ``NULL`` on invalid input.
 
    .. versionadded:: 3.2