Merge branch 'better-explain-pipeline-source' into 'master'

Add better explanation for CI_PIPELINE_SOURCE

See merge request gitlab-org/gitlab-ce!18111

doc/ci/variables/README.md

@@ -61,7 +61,7 @@ future GitLab releases.**
 | **CI_RUNNER_EXECUTABLE_ARCH**   | all    | 10.6   | The OS/architecture of the GitLab Runner executable (note that this is not necessarily the same as the environment of the executor) |
 | **CI_PIPELINE_ID**              | 8.10   | 0.5    | The unique id of the current pipeline that GitLab CI uses internally |
 | **CI_PIPELINE_TRIGGERED**       | all    | all    | The flag to indicate that job was [triggered] |
-| **CI_PIPELINE_SOURCE**          | 10.0   | all    | The source for this pipeline, one of: push, web, trigger, schedule, api, external. Pipelines created before 9.5 will have unknown as source |
+| **CI_PIPELINE_SOURCE**          | 10.0   | all    | Indicates how the pipeline was triggered. Possible options are: `push`, `web`, `trigger`, `schedule`, `api`, and `pipeline`. For pipelines created before GitLab 9.5, this will show as `unknown` |
 | **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 |
 | **CI_PROJECT_NAME**             | 8.10   | 0.5    | The project name that is currently being built (actually it is project folder name) |