Add some clarifications to Geo Registry replication doc

doc/administration/geo/replication/docker_registry.md

@@ -51,7 +51,7 @@ We need to make Docker Registry send notification events to the
        'threshold' => 5,
        'backoff' => '1s',
        'headers' => {
-         'Authorization' => ['<replace_with_a_secret_token>']
+         'Authorization' => ['<replace_with_a_secret_token>'] # An alphanumeric string. Case sensitive and must start with a letter.
        }
      }
    ]
@@ -59,7 +59,7 @@ We need to make Docker Registry send notification events to the
 
    NOTE: **Note:**
    If you use an external Registry (not the one integrated with GitLab), you must add
-   these settings to its configuration. In this case, you will also have to specify
+   these settings to its configuration yourself. In this case, you will also have to specify
    notification secret in `registry.notification_secret` section of
    `/etc/gitlab/gitlab.rb` file.
 
@@ -100,7 +100,7 @@ generate a short-lived JWT that is pull-only-capable to access the
 
    ```ruby
    gitlab_rails['geo_registry_replication_enabled'] = true
-   gitlab_rails['geo_registry_replication_primary_api_url'] = 'http://primary.example.com:5000/' # internal address to the primary registry, will be used by GitLab to directly communicate with primary registry API
+   gitlab_rails['geo_registry_replication_primary_api_url'] = 'http://primary.example.com:4567/' # Primary registry address, it will be used by the secondary node to directly communicate to primary registry
    ```
 
 1. Reconfigure the **secondary** node for the change to take effect: