docs(emscripten): link panic=unwind tracking issue

Per review feedback on wasm-bindgen#1583, link to the upstream tracking issue at
wasm-bindgen#5165 so readers can see when this limitation is lifted.

Author: guybedford

docs/src/emscripten-target.md

@@ -88,7 +88,8 @@ Two things are notable compared to the default template:
 2. **`.cargo/config.toml`** — selects the emscripten target and sets the
    rustflags emcc expects:
    - `-Cpanic=abort` — `panic=unwind` isn't supported across the wasm-bindgen
-     boundary on emscripten yet.
+     boundary on emscripten yet
+     ([wasm-bindgen#5165](https://github.com/wasm-bindgen/wasm-bindgen/issues/5165)).
    - `-Crelocation-model=static` — the staticlib is linked directly; PIC
      isn't needed.
    - `-Cllvm-args=-enable-emscripten-cxx-exceptions=0` — avoids pulling in