Add bullet point asking whether pull request is addressing a single logical change

### What problem does your feature request solve?

Pull requests being created often contain many different changes, making them large and hard to review.

### Describe the solution you'd like

Add bullet point to the development checklist asking whether pull request is addressing a single logical change, e.g. a single diagnostic.

### Describe alternatives you've considered

Find the development checklist referenced by the issue and read its surrounding guidance. Add a bullet asking whether the pull request addresses a single logical change, such as a single diagnostic. Done means the checklist contains this question in a clear, consistent format.

Written by the indexing model from the issue text.