docs: document both head and merge ref formats for MR pipelines

Kai Armstrongrequested to merge
docs/job-artifact-mr-ref-formats into main

What does this MR do and why?

Updates the glab job artifact help text to document both merge request ref formats (/head and /merge), since the correct format depends on which type of pipeline you want:

  • refs/merge-requests/<iid>/head: pipeline on the source branch HEAD
  • refs/merge-requests/<iid>/merge: pipeline on the merged result (pipelines for merged results)

Previously only /head was documented, which led to user confusion when their pipelines used the /merge format.

Closes https://gitlab.com/gitlab-org/gitlab/-/work_items/598563

MR acceptance checklist

  • This MR does not have a documentation impact
  • Changelog entry added

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