Add docs and tests for unary op on class (#1814)

diff --git a/docs/advanced/classes.rst b/docs/advanced/classes.rst
index c9a0da5..ae5907d 100644
--- a/docs/advanced/classes.rst
+++ b/docs/advanced/classes.rst
@@ -662,6 +662,7 @@
             .def(py::self *= float())
             .def(float() * py::self)
             .def(py::self * float())
+            .def(-py::self)
             .def("__repr__", &Vector2::toString);
     }