We now have a rather large section for feature reference, and I don't think
there is anything in this page that isn't covered elsewhere and in more detail
anymore.
This commit implements the `extends` attribute for `#[wasm_bindgen]` to
statically draw the inheritance hierarchy in the generated bindings, generating
appropriate `AsRef`, `AsMut`, and `From` implementations.
* Add missing documentation for a couple existing flags.
* Make each flag its own header, so it gets an anchor, and can be permalinked
to.
* Streamline some content and descriptions.
