[wasm-shape] Comment re thread-safety

src/hb-wasm-shape.cc

@@ -114,7 +114,14 @@ struct hb_wasm_face_data_t {
 static bool
 _hb_wasm_init ()
 {
-  /* XXX Umm. Make this threadsafe. How?! */
+  /* XXX
+   *
+   * Umm. Make this threadsafe. How?!
+   * It's clunky that we can't allocate a static mutex.
+   * So we have to first allocate one on the heap atomically...
+   *
+   * Also, wasm-micro-runtime uses a singleton instance. So if
+   * another library or client uses it, all bets are off. :-( */
 
   static bool initialized;
   if (initialized)