doc: note limitations for webr

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