Clarify CI platform comment

Author: ncoghlan

.github/workflows/test.yml

@@ -41,7 +41,8 @@ jobs:
       max-parallel: 8
       matrix:
         python-version: ["3.10", "3.11", "3.12", "3.13"]
-        # There's no platform specific SDK code, but explicitly check Windows anyway
+        # There's no platform specific SDK code, but explicitly check Windows
+        # to ensure there aren't any inadvertent POSIX-only assumptions
         os: [ubuntu-22.04, windows-2022]
 
     # Check https://github.com/actions/action-versions/tree/main/config/actions