f - clarify sender of tx_init_rbf becomes initiator of interactive-tx

Author: jkczyz

02-peer-protocol.md

@@ -1408,6 +1408,9 @@ a transaction paying more fees to make the channel confirm faster.
 
 The sender of `tx_init_rbf`:
   - MAY be either the *initiator* or the *accepter*
+    - If the sender is the accepter, it becomes the initiator of the `interactive-tx` session and thus:
+      - MUST send `tx_add_output` for the channel output
+      - MUST pay the fees for the shared transaction fields
   - MUST NOT have sent or received a `channel_ready` message.
 
 The recipient: