Changelog: Add info about chat pagination fix.

CHANGELOG.md

@@ -28,6 +28,8 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Removed `:managed_config` option. In practice, it was accidentally removed with 2.0.0 release when frontends were
 switched to a new configuration mechanism, however it was not officially removed until now.
 
+### Fixed
+- Add documented-but-missing chat pagination.
 
 ## [2.1.2] - 2020-09-17