Add extra information to state change notes

What does this MR do and why?

This MR updates merged state change note when related MRs get marked as merged when its diff_head_sha is included in another MR which just got merged. Users are getting confused as we currently just say merged regardless of why we mark them as merged. Only the MRs that get merged via GitLab UI should have a note that just say merged after this change.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

When merging of another MR via GitLab UI triggered the merge:

When merging manually outside of GitLab UI triggered merge:

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Related to #436061 (closed)