Update CHANGELOG.md

Also mention paritytech/polkadot-sdk#3844

Author: ggwpez

CHANGELOG.md

@@ -84,6 +84,7 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 #### From [#322](https://github.com/polkadot-fellows/runtimes/pull/322):
 
+- The `MessageQueue` also runs "on idle", this causes `MessageQueue::Processed` events to be emitted in other phases than just initialization ([SDK v1.13 #3844](https://github.com/paritytech/polkadot-sdk/pull/3844)).
 - AdaptPrice trait is now price controlled ([SDK v1.13 #4521](https://github.com/paritytech/polkadot-sdk/pull/4521)).
 - Allow StakingAdmin to manage nomination pool configurations ([SDK v1.11 #3959](https://github.com/paritytech/polkadot-sdk/pull/3959)).
 - Bridge: make some headers submissions free ([SDK v1.12 #4102](https://github.com/paritytech/polkadot-sdk/pull/4102)).