docs: clean up PR checklist

[hoffa]

Issue #, if available

Description of changes

• No need to test intrinsic functions; per the development guidelines we longer do it explicitly. Instead AWS::LanguageExtensions resolves them beforehand.
• Verifying the transformed template deploys and works as expected is the integration test.
• Documentation changes are done once the transform is deployed.
• The last check (or, 3 nested checks) is a good intention about not breaking backward compatibility (in development guidelines), but it's verbose and confusing in its current state.

Description of how you validated changes

Checklist

• Adheres to the development guidelines
• Add/update transform tests
  • Using correct values
  • Using wrong values
• Add/update integration tests

Examples?

Please reach out in the comments if you want to add an example. Examples will be
added to sam init through aws/aws-sam-cli-app-templates.

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

[acristin]

Seems good to me