doc: document additional session ID / run ID requirements

Based on a conversation with @hellais.

DESIGN.md

@@ -525,7 +525,9 @@ idea anyway.
 and we concluded that we need more inter-team discussion. A session
 could run for longer time than a single measurement session, but that
 may leak the users' behavior. Additionally, it may be that the probe ID
-already provides a useful signal there.
+already provides a useful signal there. However, it seems that the probe
+ID is not being ready for around one year, so an interim solution
+could be a nonce generated by the probe as an annotation and called run ID.
 
 ## Handwaving: running telegram with no backend support