Document docs cache preview pin

Author: wan9chi

.github/workflows/deploy-docs-preview.yml

@@ -33,6 +33,7 @@ jobs:
 
       - uses: voidzero-dev/setup-vp@2dec1e33f4ab2c6d5bce1b0c4607961bb1a3f7a1 # v1.12.0
         env:
+          # Use a pkg.pr.new Vite+ build while validating docs task cache behavior.
           VP_PR_VERSION: 0fecc75a64a39bc5600ff6a1cee8baff378a8cc1
         with:
           cache: true