Document or list/reference `ImageData` and other missing public APIs and further clarify which are non-standard

## Feature

Could the API for `createImageData` be replaced by or supplemented by `ImageData` for slightly easier parity with the [browser constructor](https://developer.mozilla.org/en-US/docs/Web/API/ImageData/ImageData)?

## Steps to Produce

```js
const {ImageData} = require('canvas');
const data = new ImageData(200, 200);
```

## Your Environment
* Version of node-canvas (output of `npm list canvas` or `yarn list canvas`): 2.6.0
* Environment (e.g. node 4.2.0 on Mac OS X 10.8): 10.16.3, Mac OS X 10.14.6