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".

docs/advanced/pycpp/numpy.rst

@@ -190,7 +190,7 @@ expects the type followed by field names:
     };
 
     // ...
-    PYBIND11_PLUGIN(test) {
+    PYBIND11_MODULE(test, m) {
         // ...
 
         PYBIND11_NUMPY_DTYPE(A, x, y);
@@ -284,10 +284,8 @@ simply using ``vectorize``).
         return result;
     }
 
-    PYBIND11_PLUGIN(test) {
-        py::module m("test");
+    PYBIND11_MODULE(test, m) {
         m.def("add_arrays", &add_arrays, "Add two NumPy arrays");
-        return m.ptr();
     }
 
 .. seealso::