Update PR template

Stress an API change is enough to warrant a `changelog.d` entry.
Check haskell#9856 to see why we need it.

.github/pull_request_template.md

@@ -2,7 +2,7 @@ Please read [Github PR Conventions](https://github.com/haskell/cabal/blob/master
 
 ---
 
-**Template Α: This PR modifies `cabal` behaviour**
+**Template Α: This PR modifies `cabal` behaviour *or* the API of any `cabal` package**
 
 Include the following checklist in your PR:
 
@@ -14,10 +14,11 @@ Include the following checklist in your PR:
 
 ---
 
-**Template Β: This PR does not modify `cabal` behaviour (documentation, tests, refactoring, etc.)**
+**Template Β: This PR does not modify `cabal` behaviour or the API of any `cabal` package**
+
+*(e.g.: documentation, tests, refactoring, etc.)*
 
 Include the following checklist in your PR:
 
 * [ ] Patches conform to the [coding conventions](https://github.com/haskell/cabal/blob/master/CONTRIBUTING.md#other-conventions).
 * [ ] Is this a PR that fixes CI? If so, it will need to be backported to older cabal release branches (ask maintainers for directions).
-