f: only allow traffic mimicing if set in invoice

Author: carlaKC

recommendations/local-resource-conservation.md

@@ -84,7 +84,8 @@ as resolution is ultimately dependent on their hold time.
 
 The original sender:
 - SHOULD NOT set `accountable` in their offered `update_add_htlc`.
-- MAY set `accountable` to mimic patterns observed in their forwarded traffic.
+- MAY set `accountable` to mimic patterns observed in their forwarded traffic
+  if `upgrade_accountability` is set in the recipient's invoice.
 
 The forwarding node:
 - If `accountable` is set in the received `update_add_htlc`: