Merge branch 'docs/triggers' into 'master'

Refactor CI triggers docs

Closes #2616

See merge request !2079

doc/ci/variables/README.md

@@ -53,7 +53,7 @@ future GitLab releases.**
 | **CI_RUNNER_ID**                | 8.10   | 0.5    | The unique id of runner being used |
 | **CI_RUNNER_TAGS**              | 8.10   | 0.5    | The defined runner tags |
 | **CI_PIPELINE_ID**              | 8.10   | 0.5    | The unique id of the current pipeline that GitLab CI uses internally |
-| **CI_PIPELINE_SOURCE**          | 9.3    | all    | The variable indicates how the pipeline was triggered, possible options are: push, web, trigger, schedule, api, pipeline |
+| **CI_PIPELINE_SOURCE**          | 9.3    | all    | ([EEP]) The variable indicates how the pipeline was triggered, possible options are: push, web, trigger, schedule, api, pipeline |
 | **CI_PIPELINE_TRIGGERED**       | all    | all    | The flag to indicate that job was [triggered] |
 | **CI_PROJECT_DIR**              | all    | all    | The full path where the repository is cloned and where the job is run |
 | **CI_PROJECT_ID**               | all    | all    | The unique id of the current project that GitLab CI uses internally |
@@ -430,3 +430,4 @@ export CI_REGISTRY_PASSWORD="longalfanumstring"
 [protected branches]: ../../user/project/protected_branches.md
 [protected tags]: ../../user/project/protected_tags.md
 [shellexecutors]: https://docs.gitlab.com/runner/executors/
+[eep]: https://about.gitlab.com/gitlab-ee/ "Available only in GitLab Enterprise Edition Premium"