Update docs/advanced/exceptions.rst

Co-authored-by: Henry Schreiner <HenrySchreinerIII@gmail.com>

docs/advanced/exceptions.rst

@@ -338,7 +338,7 @@ exceptions was introduced:
 
 To do a similar thing in pybind11, you can use the ``py::raise_from`` function. It
 sets the current python error indicator, so to continue propagating the exception
-you should ``throw py::error_already_set()``.
+you should ``throw py::error_already_set()`` (Python 3 only).
 
 .. code-block:: cpp