gitlab slash commands is broken or docs too convoluted to understand

The official docs just give example how to configure, but no working example is provided: https://docs.gitlab.com/ce/user/project/integrations/slack_slash_commands.html

searching doesn't yield anything useful either, perhaps just one match only: http://julianhigman.com/blog/2017/10/19/deploying-git-branches-from-gitlab-using-slack-commands/

however, using the example given slack keeps responding with Couldn't find a deployment manual action. and that's it.

stages:
  - test
  - deploy

test:
  stage: test
  environment: $CI_COMMIT_REF_NAME
  script:
    - ./bin/test.sh

deploy:
  stage: deploy
  environment: $CI_COMMIT_REF_NAME
  when: manual
  script:
    - ./bin/deploy_to_server.sh $CI_COMMIT_REF_NAME

please make documentation useful, provide working example and/or explain better what "source environment" or "target enviroment" mean or how to troubleshoot.

Unknown command, these commands are available
- /test issue show <id>
- /test issue new <title> *⇧ Shift*+*↵ Enter* <description>
- /test issue search <your query>
- /test deploy <environment> to <target-environment>