Improve Praefect db guidance

The previous guidance used database when it meant server. This meant it
was ambiguous what the recommended configuration is, and there was no
explaination for why this is the recommended configuration.

doc/administration/gitaly/praefect.md

@@ -98,8 +98,10 @@ We will note in the instructions below where these secrets are required.
 
 ### PostgreSQL
 
-NOTE: **Note:** don't reuse the GitLab application database for the Praefect
-database.
+NOTE: **Note:** do not store the GitLab application database and the Praefect
+database on the same PostgreSQL server if using
+[Geo](../geo/replication/index.md). The replication state is internal to each instance
+of GitLab and should not be replicated.
 
 To complete this section you will need: