Merge branch 'add-slack-app-case-to-docs' into 'master'

Add slack app case to docs

See merge request gitlab-org/gitlab-ce!15729

doc/integration/slash_commands.md

@@ -17,6 +17,9 @@ Taking the trigger term as `project-name`, the commands are:
 | `/project-name issue search <query>` | Shows up to 5 issues matching `<query>` |
 | `/project-name deploy <from> to <to>` | Deploy from the `<from>` environment to the `<to>` environment |
 
+Note that if you are using the [GitLab Slack application](https://docs.gitlab.com/ee/user/project/integrations/gitlab_slack_application.html) for 
+your GitLab.com projects, you need to [add the `gitlab` keyword at the beginning of the command](https://docs.gitlab.com/ee/user/project/integrations/gitlab_slack_application.html#usage).
+
 ## Issue commands
 
 It is possible to create new issue, display issue details and search up to 5 issues.