Improve cluster sizing doc on control plane/metastore/janitor. Closes quickwit-oss#5872 (quickwit-oss#6202)

Author: osyniakov

docs/deployment/cluster-sizing.md

@@ -74,17 +74,22 @@ Searcher nodes based on:
 ### Other services
 
 The Control Plane, Metastore and, Janitor are lightweight components.
-Each of these services requires 1 replica.
 
-The Control Plane needs a single core and 2GB of RAM. It doesn't require any disk.
-
-The Metastore also requires a single core and 2GB of RAM. For clusters handling
-hundreds of indexes, you may increase the size to 2 cores and 4GB of RAM. It
-doesn't write to disk.
-
-In general, the Janitor requires 1 core and 2GB of RAM and doesn't use the disk.
-If you use the [delete API](https://quickwit.io/docs/overview/concepts/deletes),
-the Janitor should be dimensioned like an indexer.
+- **Control Plane**: A cluster must have only one Control Plane. It needs a
+  single core and 2GB of RAM. It doesn't require any disk.
+
+- **Metastore**: A cluster must have exactly one Metastore when using the
+  [file-backed metastore](../configuration/metastore-config.md#file-backed-metastore).
+  When using the [PostgreSQL metastore](#postgres-metastore-backend), you can
+  run one or several Metastore pods for high availability (HA). The Metastore
+  requires a single core and 2GB of RAM. For clusters handling hundreds of
+  indexes, you may increase the size to 2 cores and 4GB of RAM. It doesn't
+  write to disk (when using PostgreSQL, the database handles persistence).
+
+- **Janitor**: A cluster must have only one Janitor. In general, it requires 1
+  core and 2GB of RAM and doesn't use the disk. If you use the [delete
+  API](https://quickwit.io/docs/overview/concepts/deletes), the Janitor should
+  be dimensioned like an indexer.
 
 ### Single node deployments