Add Py_UNICODE_REPLACEMENT_CHARACTER

Author: StanFromIreland

Doc/c-api/unicode.rst

@@ -193,6 +193,14 @@ are available through these macros which are mapped to C functions depending on
 the Python configuration.
 
 
+.. c:macro:: Py_UNICODE_REPLACEMENT_CHARACTER
+
+   The Unicode code point ``U+FFFD`` (REPLACEMENT CHARACTER).
+
+   This Unicode character is used as the replacement character during
+   decoding if the errors argument is set to "replace".
+
+
 .. c:function:: int Py_UNICODE_ISSPACE(Py_UCS4 ch)
 
    Return ``1`` or ``0`` depending on whether *ch* is a whitespace character.