Merge branch 'docs-set-service-variables-tip' into 'master'

Add notes to the docs about setting service variables for users wanting to do so from the UI

See merge request gitlab-org/gitlab-ce!30863

doc/ci/services/mysql.md

@@ -25,6 +25,12 @@ variables:
   MYSQL_ROOT_PASSWORD: "<your_mysql_password>"
 ```
 
+NOTE: **Note:**
+The `MYSQL_DATABASE` and `MYSQL_ROOT_PASSWORD` variables can't be set in the GitLab UI.
+To set them, assign them to a variable [in the UI](../variables/README.md#via-the-ui), 
+and then assign that variable to the 
+`MYSQL_DATABASE` and `MYSQL_ROOT_PASSWORD` variables in your `.gitlab-ci.yml`.
+
 And then configure your application to use the database, for example:
 
 ```yaml

doc/ci/services/postgres.md

@@ -25,6 +25,13 @@ variables:
   POSTGRES_PASSWORD: ""
 ```
 
+NOTE: **Note:**
+The `POSTGRES_DB`, `POSTGRES_USER`, and `POSTGRES_PASSWORD` variables can't be set in
+the GitLab UI. To set them, assign them to a variable
+[in the UI](../variables/README.md#via-the-ui), and then assign that
+variable to the `POSTGRES_DB`, `POSTGRES_USER`, and `POSTGRES_PASSWORD` variables in
+your `.gitlab-ci.yml`.
+
 And then configure your application to use the database, for example:
 
 ```yaml