changelog: adds note about breaking change (grafana#22480)

CHANGELOG.md

@@ -3,6 +3,8 @@
 ## Breaking changes
 * **Slack**: Removed _Mention_ setting and instead introduce _Mention Users_, _Mention Groups_, and _Mention Channel_. The first two settings require user and group IDs, respectively. This change was necessary because the way of mentioning via the Slack API [changed](https://api.slack.com/changelog/2017-09-the-one-about-usernames) and mentions in Slack notifications no longer worked.
 
+* **Alerting**: Reverts the behavior of `diff` and `percent_diff` to not always be absolute. Something we introduced by mistake in [6.1.0](https://github.com/grafana/grafana/commit/28eaac3a9c7082e8c496005c1cb66b4b70a4f82f). Alerting now support `diff()`, `diff_abs()`, `percent_diff()` and `percent_diff_abs()`. [#21338](https://github.com/grafana/grafana/pull/21338)
+
 ### Notice about changes in backendSrv for plugin authors
 
 In our mission to migrate away from AngularJS to React we have removed all AngularJS dependencies in the core data retrieval service `backendSrv`.