Add link to fixing prometheus to 1.32.0 upgrade notes; 1.32.1 has a fix

CHANGES.md

 Synapse 1.32.1 (2021-04-21)
 ===========================
+This release fixes the regression introduced in Synapse 
+
 Bugfixes
 --------

UPGRADE.rst

@@ -94,8 +94,10 @@ Regression causing connected Prometheus instances to become overwhelmed
 This release introduces `a regression <https://github.com/matrix-org/synapse/issues/9853>`_
 that can overwhelm connected Prometheus instances. This issue was not present in
 Synapse v1.32.0rc1. It is recommended not to update to this release. If you have
-upgraded to v1.32.0 already, please downgrade to v1.31.0. This issue will be
-resolved in a subsequent release version shortly.
+upgraded to v1.32.0 already, please upgrade to v1.31.1 which contains a fix.
+If you started Synapse on v1.32.0, you may need to remove excess writeahead logs
+in order for Prometheus to recover; instructions for doing so are
+`here <https://github.com/matrix-org/synapse/pull/9854#issuecomment-823472183>`_.
 
 Dropping support for old Python, Postgres and SQLite versions
 -------------------------------------------------------------