dual-funding: note about when we switch from temp to 'real' channel_id

for v2 channel establishment we switch from the temporary channel_id
to the channel_id derived from the funding_txid after both
funding_compose messages have been exchanged, as at this point
both opener/accepter have the ability to derive the funding_txid
which is required for the channel_id.

02-peer-protocol.md

@@ -679,6 +679,9 @@ The receiving node:
 
 The first commitment transaction has no HTLC's in it.
 
+Note that the `commitment_signed` message will include the `channel_id` derived from
+the `funding_txid`, instead of the `temporary_node_id`.
+
 ### The `funding_signed2` Message
 
 This message exchanges the witness data for the inputs that were