docs: Add bsr key to requirements

website/content/docs/configuration/session-recording/enable-session-recording.mdx

@@ -13,8 +13,11 @@ You use the storage bucket's ID to associate a target with the storage bucket.
 
 **Requirements**:
 
-- One or more storage buckets to store the recordings
+- One or more storage buckets to store the recordings.
 - Session recording is only supported for SSH targets at this time.
+- A KMS key must be added to the controller configuration with the purpose `bsr`.
+The key is used for encrypting data and checking the integrity of recordings.
+Refer to [Create the controller configuration](/boundary/docs/install-boundary/configure-controllers#create-the-controller-configuration) for more information about configuring a KMS block.
 - The targets must be configured with an ingress or egress worker filter that includes a worker with access to the storage bucket you created.
 Refer to [SSH target attributes](/boundary/docs/concepts/docmain-model/targets#ssh-target-attributes-hcp-ent) for more information.
 - You must enable injected application credentials on any target that you want to use for session recording.