Replace PYBIND11_PLUGIN with PYBIND11_MODULE

This commit also adds `doc()` to `object_api` as a shortcut for the
`attr("__doc__")` accessor.

The module macro changes from:
```c++
PYBIND11_PLUGIN(example) {
    pybind11::module m("example", "pybind11 example plugin");
    m.def("add", [](int a, int b) { return a + b; });
    return m.ptr();
}
```

to:

```c++
PYBIND11_MODULE(example, m) {
    m.doc() = "pybind11 example plugin";
    m.def("add", [](int a, int b) { return a + b; });
}
```

Using the old macro results in a deprecation warning. The warning
actually points to the `pybind11_init` function (since attributes
don't bind to macros), but the message should be quite clear:
"PYBIND11_PLUGIN is deprecated, use PYBIND11_MODULE".
diff --git a/docs/advanced/cast/functional.rst b/docs/advanced/cast/functional.rst
index 5d0a01d..d9b4605 100644
--- a/docs/advanced/cast/functional.rst
+++ b/docs/advanced/cast/functional.rst
@@ -56,14 +56,10 @@
 
     #include <pybind11/functional.h>
 
-    PYBIND11_PLUGIN(example) {
-        py::module m("example", "pybind11 example plugin");
-
+    PYBIND11_MODULE(example, m) {
         m.def("func_arg", &func_arg);
         m.def("func_ret", &func_ret);
         m.def("func_cpp", &func_cpp);
-
-        return m.ptr();
     }
 
 The following interactive session shows how to call them from Python.