Specify delay_id grammar

proposals/4140-delayed-events-futures.md

@@ -126,6 +126,8 @@ Content-Type: application/json
 }
 ```
 
+The `delay_id` is generated by the server. It SHOULD contain only valid non-surrogate Unicode codepoints except `:` and `NUL`, and MUST NOT exceed 255 bytes.
+
 The server MAY round the delay up to a maximum of 30 seconds away from the request.
 
 If the requested delay exceeds the maximum delay defined by the homeserver, the server will respond with HTTP 400,