Mention removal of bare multisig IsMine in release notes

doc/release-notes.md

@@ -96,6 +96,16 @@ Low-level RPC changes
   with any `-wallet=<path>` options, there is no change in behavior, and the
   name of any wallet is just its `<path>` string.
 
+- Bare multisig outputs to our keys are no longer automatically treated as
+  incoming payments. As this feature was only available for multisig outputs for
+  which you had all private keys in your wallet, there was generally no use for
+  them compared to single-key schemes. Furthermore, no address format for such
+  outputs is defined, and wallet software can't easily send to it. These outputs
+  will no longer show up in `listtransactions`, `listunspent`, or contribute to
+  your balance, unless they are explicitly watched (using `importaddress` or
+  `importmulti` with hex script argument). `signrawtransaction*` also still
+  works for them.
+
 ### Logging
 
 - The log timestamp format is now ISO 8601 (e.g. "2018-02-28T12:34:56Z").