package/ci: move a comment to where the macOS image is defined.

package/ci/circleci.yml

@@ -17,6 +17,9 @@ executors:
     docker:
     - image: ubuntu:bionic-20200921
   xcode-11_2:
+    # Molten-vk isn't in (non-updated) Homebrew on the 9.4 or 10.0/1/2 image,
+    # have to use 10.3 instead; since 2021-06-08 it refuses to work on 10.14 so
+    # have to use 11.2 at least
     macos:
       xcode: 11.2.1
   xcode-11_6:
@@ -391,9 +394,6 @@ jobs:
         script: unix-desktop.sh
 
   macos-gl:
-    # Molten-vk isn't in (non-updated) Homebrew on the 9.4 or 10.0/1/2 image,
-    # have to use 10.3 instead; since 2021-06-08 it refuses to work on 10.14 so
-    # have to use 11.2 at least
     executor: xcode-11_2
     environment:
       CMAKE_CXX_FLAGS: --coverage