src: add comment on CallbackScope exception behaviour

PR-URL: #21743
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Shelley Vohr <codebytere@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>

src/node.h

@@ -683,6 +683,10 @@ class InternalCallbackScope;
  *
  * This object should be stack-allocated to ensure that it is contained in a
  * valid HandleScope.
+ *
+ * Exceptions happening within this scope will be treated like uncaught
+ * exceptions. If this behaviour is undesirable, a new `v8::TryCatch` scope
+ * needs to be created inside of this scope.
  */
 class NODE_EXTERN CallbackScope {
  public: