Use new style __init__ in numpy docs

[bstaletic]

Fixes #1205

[YannickJadoul]

Just some docs. Merging. Thanks, Boris! :-)

[YannickJadoul]

@bstaletic I know I actually reviewed this, but does it work?

Should it not actually be

        .def(py::init([](py::buffer b) {
            // ...
            return new Matrix(map);
        }));

[bstaletic]

It should be map, not m. I have no idea why I made that change. However, you don't have to return a pointer. You can return a value. Do you want me to create a new pr to fix this?

[YannickJadoul]

You can return a value.

Yes, but new is called internally anyway? But as you wish. Are Matrix objects cheaply movable?

Do you want me to create a new pr to fix this?

Up to you. I can also do it; I have it ready.

[YannickJadoul]

It should be map, not m. I have no idea why I made that change.

I must also have been sleeping during that review, though :-/