clarify iOS sending "New message"

	closes https://github.com/binwiederhier/ntfy/issues/465

docs/config.md

@@ -724,6 +724,8 @@ curl -X POST -H "X-Poll-ID: s4PdJozxM8na" https://ntfy.sh/6de73be8dfb7d69e32fb2c
 {"id":"4HsClFEuCIcs","time":1654087955,"event":"poll_request","topic":"6de73be8dfb7d69e32fb2c00c23fe7adbd8b5504406e3068c273aa24cef4055b","message":"New message","poll_id":"s4PdJozxM8na"}
 ```
 
+The self-hosted server literally sends the message "New message" for every message. Even if your message is "Some other message", the message will always be "New message". This is so that if iOS cannot talk to the selfhosted server (in time, or at all), it'll show "New message" as a popup.
+
 ## Rate limiting
 !!! info
     Be aware that if you are running ntfy behind a proxy, you must set the `behind-proxy` flag.