Expand on why users should read upgrade notes

CHANGES.md

 Synapse 1.42.0rc2 (2021-09-06)
 ==============================
-Server administrators are reminded to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
+This version of Synapse removes deprecated room-management admin APIs and out-of-date
+email pushers, and improves error handling for fallback templates for user-interactive
+authentication. For more information on these points, server administrators are
+encouraged to read [the upgrade notes](docs/upgrade.md#upgrading-to-v1420).
 Features
 --------