Document "reconnect ad-infinitum" SSE pitfall.

core/lib/src/response/stream/sse.rs

@@ -475,6 +475,16 @@ impl Event {
 ///
 /// To send messages losslessly, they must be encoded first, for instance, by
 /// using [`Event::json()`].
+///
+/// * **Clients reconnect ad-infinitum**
+///
+/// The [SSE standard] stipulates: _"Clients will reconnect if the connection
+/// is closed; a client can be told to stop reconnecting using the HTTP 204
+/// No Content response code."_ As a result, clients will typically reconnect
+/// exhaustively until either they choose to disconnect or they receive a
+/// `204 No Content` response.
+///
+/// [SSE standard]: https://html.spec.whatwg.org/multipage/server-sent-events.html
 pub struct EventStream<S> {
  stream: S,
  heartbeat: Option<Duration>,