Clarify upstream CTK version control in versions.yml

Added comments to clarify version control for upstream CTK.

Author: mdboom

ci/versions.yml

@@ -3,11 +3,15 @@
 
 backport_branch: "12.9.x"  # keep in sync with target-branch in .github/dependabot.yml
 
+# This controls the version of the upstream CTK to install to build the wheels.
+# It is not the version of cuda_bindings/cuda_python being released, which is 
+# solely determined by git tags via setuptools_scm.
 cuda:
   build:
     version: "13.4.2"
   prev_build:
     version: "12.9.1"
+    
 python:
   # Python versions the wheel matrix is built across. ci.yml reads this list
   # and passes it to build-wheel.yml as the `python-versions` input.