Update PR template

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

Author: ffaf1

.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:
 
@@ -14,7 +14,7 @@ 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 `Cabal` API (documentation, tests, refactoring, etc.)**
 
 Include the following checklist in your PR: