Mention Cloudflare in the comment

rust/src/rendezvous/mod.rs

@@ -292,6 +292,7 @@ impl RendezvousHandler {
         // Even though this isn't mandated by the MSC, we set a Content-Type on the response. It
         // doesn't do any harm as the body is empty, but this helps escape a bug in some reverse
         // proxy/cache setup which strips the ETag header if there is no Content-Type set.
+        // Specifically, we noticed this behaviour when placing Synapse behind Cloudflare.
         response.headers_mut().typed_insert(ContentType::text());
 
         http_response_to_twisted(twisted_request, response)?;