[docs] Add release notes for removal of REJECT reasons

doc/release-notes-15437.md

@@ -32,3 +32,22 @@ Please use the recommended alternatives if you rely on this deprecated feature:
   could wait until the transaction has confirmed (taking into account the fee
   target they set (compare the RPC `estimatesmartfee`)) or listen for the
   transaction announcement by other network peers to check for propagation.
+
+The removal of BIP61 REJECT message support also has the following minor RPC
+and logging implications:
+
+* `testmempoolaccept` and `sendrawtransaction` no longer return the P2P REJECT
+  code when a transaction is not accepted to the mempool. They still return the
+  verbal reject reason.
+
+* Log messages that previously reported the REJECT code when a transaction was
+  not accepted to the mempool now no longer report the REJECT code. The reason
+  for rejection is still reported.
+
+Updated RPCs
+------------
+
+- `testmempoolaccept` and `sendrawtransaction` no longer return the P2P REJECT
+  code when a transaction is not accepted to the mempool. See the Section
+  _Removal of reject network messages from Bitcoin Core (BIP61)_ for details on
+  the removal of BIP61 REJECT message support.