Adds more info about user_session_logins_total in docs

doc/administration/monitoring/prometheus/gitlab_metrics.md

@@ -101,7 +101,7 @@ The following metrics are available:
 | `http_elasticsearch_requests_total` **(STARTER)**              | Counter   |                   13.1 | Elasticsearch requests count during web transactions                                                | `controller`, `action`                              |
 | `pipelines_created_total`                                      | Counter   |                    9.4 | Counter of pipelines created                                                                        |                                                     |
 | `rack_uncaught_errors_total`                                   | Counter   |                    9.4 | Rack connections handling uncaught errors count                                                     |                                                     |
-| `user_session_logins_total`                                    | Counter   |                    9.4 | Counter of how many users have logged in                                                            |                                                     |
+| `user_session_logins_total`                                    | Counter   |                    9.4 | Counter of how many users have logged in since GitLab was started or restarted                                                            |                                                     |
 | `upload_file_does_not_exist`                                   | Counter   | 10.7 in EE, 11.5 in CE | Number of times an upload record could not find its file                                            |                                                     |
 | `failed_login_captcha_total`                                   | Gauge     |                   11.0 | Counter of failed CAPTCHA attempts during login                                                     |                                                     |
 | `successful_login_captcha_total`                               | Gauge     |                   11.0 | Counter of successful CAPTCHA attempts during login                                                 |                                                     |