doc: note limitations for webr

[JosiahParry]

webr support has two key limitations:

1. if you're using rayon you must either ensure that the thread pool used is only the main thread otherwise the package will not work
2. all dependencies must be compatible with the wasm32-unknown-emscripten target. Some dependencies are not compatible with WASM at all, these cannot be built.

Be sure that your package matches these requirements before expecting webr builds