Update the PR template QA instructions (#6580)

## Summary

This PR tweaks the wording of the PR template QA instructions with the
goals of:
1. Make it more clear that PR authors are responsible for testing their
PRs.
2. Encouraging sufficient detail in the test descriptions.

## Checklist

- [x] _The PR has a short but descriptive title, suitable for a
changelog_
- [x] _Tests added / updated (if applicable)_
- [x] _Documentation added / updated (if applicable)_

Author: blessedcoolant

.github/pull_request_template.md

@@ -8,7 +8,7 @@
 
 ## QA Instructions
 
-<!--WHEN APPLICABLE: Describe how we can test the changes in this PR.-->
+<!--WHEN APPLICABLE: Describe how you have tested the changes in this PR. Provide enough detail that a reviewer can reproduce your tests.-->
 
 ## Merge Plan