Clarify that # refers to an issue in MR widget

This is an actionable insight from the UX Department: MRs experience async critique research (insight).

What

When an issue is mentioned/closed by the MR, we use the # notation to refer to issues. But users may not be familiar with that, and so they wouldn't understand that those refer to issues:

How

We might be able to fix this with one word: Mentions issue #0000