Follow-up from "Add use case content for CI cache key documentation"

The following discussion from !20442 (merged) should be addressed:

• @marcel.amirault started a discussion: (+1 comment)

  @mbobin Based on your great explanation, and our recent chat, I made a few more suggestions that I hope makes everything clearer.

  I think we may want to address the default case in more detail, and expand with more use cases for the prefix, but let's iterate on that in a future MR, perhaps when we expand the ci/caching/index.md doc.