Fix source text in MR related work items tooltip

What does this MR do?

The merge request sidebar shows an information icon next to links that GitLab created automatically. The tooltip on this icon always said the link comes from the description. This is wrong for closing links, because GitLab also creates them from the title and from commit messages.

This MR changes the tooltip to: "This link comes from the merge request title, description, or a commit message. To remove the link, edit the text that references it." The same text is correct for closing links and mentioned links. The text comes from the technical writer in the issue discussion: #631705 (comment 3952615384)

This is a short-term text fix. A separate backend change will record the real source of each link. After that, the tooltip can name the exact source.

Screenshots

Before	After

How to test

1. Create a merge request. Do not put a closing pattern in the description.
2. Push a commit with "Closes" and the full URL of a work item in the commit message.
3. Open the merge request. Make sure the explicit_mr_work_item_relations feature flag is on (it is on by default) and that you can edit the merge request.
4. In the sidebar, find the Closing section of the "Work items" list.
5. Hover the information icon next to the link. The tooltip shows the new text.

References

Resolves #631705 (closed)