Docs feedback: Wrong variable used in rules:if in the "Run child pipelines with merge request pipelines" section from "Downstream pipelines"

Problem to solve

The following section of the docs uses a variable wrongly: https://docs.gitlab.com/ee/ci/pipelines/downstream_pipelines.html#run-child-pipelines-with-merge-request-pipelines.

As per this comment (and verified on the docs) $CI_PIPELINE_SOURCE has the value parent_pipeline and thus can't be used to run MR children pipelines. A successful solution was to check for the presence of the variable $CI_MERGE_REQUEST_ID.

Further details

Proposal

The docs should be updated to include a working example, using $CI_MERGE_REQUEST_ID instead of $CI_PIPELINE_SOURCE.

Who can address the issue

Other links/references

Link to the docs: https://docs.gitlab.com/ee/ci/pipelines/downstream_pipelines.html#run-child-pipelines-with-merge-request-pipelines