operators should return NotImplemented given unsupported input (fixes #393)
diff --git a/docs/advanced.rst b/docs/advanced.rst
index 748f91e..ff85a4f 100644
--- a/docs/advanced.rst
+++ b/docs/advanced.rst
@@ -90,10 +90,13 @@
 
     .def("__mul__", [](const Vector2 &a, float b) {
         return a * b;
-    })
+    }, py::is_operator())
 
 This can be useful for exposing additional operators that don't exist on the
-C++ side, or to perform other types of customization.
+C++ side, or to perform other types of customization. The ``py::is_operator``
+flag marker is needed to inform pybind11 that this is an operator, which
+returns ``NotImplemented`` when invoked with incompatible arguments rather than
+throwing a type error.
 
 .. note::