chore: clarify what we expect from PR body a bit more (#34553)

Call out that we'd like a bit more text from the PR body.

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: rix0rrr

.github/PULL_REQUEST_TEMPLATE.md

@@ -10,18 +10,19 @@ Closes #<issue number here>.
 
 <!--
 What code changes did you make? 
-Have you made any important design decisions?
-What AWS use cases does this change enable? To enable the use cases, which AWS service features are utilized?
+Why do these changes address the issue?
+What alternatives did you consider and reject?
+What design decisions have you made?
 -->
 
 ### Describe any new or updated permissions being added
 
-<!-- What new or updated IAM permissions are needed to support the changes being introduced ? -->
+<!-- What new or updated IAM permissions are needed to support the changes being introduced? -->
 
 
 ### Description of how you validated changes
 
-<!--Have you added any unit tests and/or integration tests?-->
+<!-- Have you added any unit tests and/or integration tests? Did you test by hand? -->
 
 ### Checklist
 - [ ] My code adheres to the [CONTRIBUTING GUIDE](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md) and [DESIGN GUIDELINES](https://github.com/aws/aws-cdk/blob/main/docs/DESIGN_GUIDELINES.md)