Missing documentation on supported advanced DOM types

[According to the documentation of available types](https://github.com/google/closure-compiler/wiki/Types-in-the-Closure-Type-System), the only type to describe DOM elements is `Element`.

However, I went through the source code of GCC and found [this externs declarations for DOM](https://github.com/google/closure-compiler/blob/master/externs/browser/w3c_dom2.js), plus some other externs files in the same directory, where some advanced DOM types such as `HTMLDivElement` are defined, so it seems that there is support for more than just the `Element` type.

It would be very useful to add a word or two about this in the documentation/wiki - preferably [here](https://github.com/google/closure-compiler/wiki/Types-in-the-Closure-Type-System) and supply a link to the source code. So that it can be clear and easy to understand which DOM types are supported and which are not.