Clarify the actual Visual Studio version [ci skip]

.appveyor.yml

@@ -26,8 +26,9 @@ environment:
     # is limited, and compatibility issues that happen only in newer versions are rare.
     # You may test some other stuff on GitHub Actions instead.
     - build: vs
-      vs: 120
+      vs: 120 # Visual Studio 2013
       ssl: OpenSSL-v111
+      # The worker image name. This is NOT the Visual Studio version we're using here.
       APPVEYOR_BUILD_WORKER_IMAGE: Visual Studio 2015
       GEMS_FOR_TEST: ""
   RELINE_TEST_ENCODING: "UTF-8"