Add guidance on shifting issues to backlog

This came out of a session with CS on how to improve how we work together. The feedback was that while they understand we prioritize velocity over predictability, it can be quite frustrating from a customer experience standpoint to see an issue you were hoping for that was scheduled in the near future get bumped to backlog, especially with no further comment.

In these situations, we should recommend a short comment to go along with the change, to indicate why this change is being made.