Add a note for serviceuser rotation

It's good that our users know that this happens.
It's not so relevant that they know how, why and when.

Author: Reasonable-Solutions

docs/persistence/opensearch/README.md

@@ -19,4 +19,4 @@ A very powerful pattern for OpenSearch is search and filter, also known as [face
 
 Nais offers OpenSearch via [Aiven](https://aiven.io/).
 Aiven OpenSearch can be used by applications in all environments, but must be *defined* in a GCP cluster.
-All OpenSearch instances on Aiven, regardless of plan, are backed up but there are no disaster backups outside of the provider.
+All OpenSearch instances on Aiven, regardless of plan, are backed up but there are no disaster backups outside of the provider. The serviceuser for any particular OpenSearch instance gets rotated regularly by the platform.

docs/persistence/valkey/README.md

@@ -9,6 +9,8 @@ tags: [persistence, explanation, valkey, redis, services]
 Valkey is a key value database that is used for storing and querying data.
 It is a good choice for storing data that is not relational in nature and often used for caching.
 
+The serviceuser used by the valkey instance gets rotated on a regular basis.
+
 ## Related pages
 
 :dart: [Create Valkey via Application](how-to/create-application.md)