Unclear how to delete a specific hash of a cached element

Using the `project/element/hash` syntax as shown in `bst artifact --help` and the hash as shown in `bst show` does not work.

Start by reproducing the documented project/element/hash form with bst artifact --help and the hash from bst show. Trace how the artifact deletion command handles that argument and compare it with the help text. Done means a specific cached element can be deleted using the documented form, with a regression test or corrected help text.

Written by the indexing model from the issue text.