gh-148250: Mention str subclasses in PyUnicodeWriter_WriteStr() doc (#148251)

Author: vstinner

Doc/c-api/unicode.rst

@@ -1881,6 +1881,10 @@ object.
    On success, return ``0``.
    On error, set an exception, leave the writer unchanged, and return ``-1``.
 
+   To write a :class:`str` subclass which overrides the :meth:`~object.__str__`
+   method, :c:func:`PyUnicode_FromObject` can be used to get the original
+   string.
+
 .. c:function:: int PyUnicodeWriter_WriteRepr(PyUnicodeWriter *writer, PyObject *obj)
 
    Call :c:func:`PyObject_Repr` on *obj* and write the output into *writer*.