Improve messaging on Pipelines for Merged Results (merge train)

Problem to solve

We currently do not do a very good job of documenting both in product and in our documentation, what happens when you enable Pipelines for Merged Results:

• We don't note anywhere in the GitLab UI that if target has moved since the pipeline started, the MR will fail. This is a significant detail that we should display in the UI, until we have ways to work around this. (Merge Trains)
• The documentation link in the ?, points to the wrong place. It points here: https://gitlab.com/help/ci/merge_request_pipelines/index.md#pipelines-for-merge-requests. I think it should point here: https://gitlab.com/help/ci/merge_request_pipelines/index.md#pipelines-for-merged-results-premium
• In our docs, the fact that merges will fail is only noted way down as the third bullet in this section: https://gitlab.com/help/ci/merge_request_pipelines/index.md#pipelines-for-merged-results-limitations. It's sort of the fine print, and could easily be missed.

This can lead to users enabling this feature without truly knowing all that it does and it's limitations. For example I was talking to a large customer yesterday about a different topic, and they noted that they had broke all MR's recently and had to figure out why, ultimately tracing it back to this checkbox. They then had to go and tell all of their users not to enable this checkbox, as it is likely to break their projects.

Intended users

Further details

We are conducting usability testing to get answers from users about why they find the documentation misleading, and how we can improve it. See https://gitlab.com/gitlab-org/ux-research/issues/244

Proposal

• Improve documentation for pipeline for merged results based on results of https://gitlab.com/gitlab-org/ux-research/issues/244
• Fix the ? link in the Merge requests settings page to point to: https://gitlab.com/help/ci/merge_request_pipelines/index.md#pipelines-for-merged-results-premium

Permissions and Security

Documentation

Testing

What does success look like, and how can we measure that?

• The user interface communication is clear about the pipelines for merged results functionality in the settings page
  • The user can enable this feature truly knowing all that it does and it's limitations
• The relevant docs content is clear on what's possible in GitLab related to pipelines for merged results

Links / references

/label ~feature