Document all features of wasm-bindgen

[RunDevelopment]

This documents all features and puts them into 3 categories:

1. User-facing features. These are features users are allowed to use and that we actively support. E.g. strict-macro.
2. Deprecated features. These are features users are not supposed to use anymore, but nonetheless allowed.
3. Internal features. Features for testing and debugging that happen to public.

I documented all user-facing and deprecated feature and put a note on all internal features.

[daxpedda]

Great!