This command is mentioned in the online docs only very briefly ("clean" appears two times on http://docs.haskellstack.org/en/stable/GUIDE/) but its semantics is not defined.

I think "how to delete results from earlier builds/trigger a rebuild" is a basic question. I can understand that the one true answer is "you don't, because builds are deterministic", but sometimes they are not (see issue #2277).

The executable is more helpful

 stack clean --help
Usage: stack clean ([PACKAGE] | [--full]) [--help]
  Clean the local packages

Available options:
  PACKAGE                  If none specified, clean all local packages
  --full                   Delete all work directories (.stack-work by default)

but I still don't know what "clean a package" means.

My suggestion is to (keep this output, but) add a paragraph to the (online) docs. In fact, I'd expect that I would find there: a list of all stack commands, and then for each of them, a description of what it does. Instead, the User Guide is a mixture of motivation and tutorial. That's useful, but it leaves some questions open. (Yes, I know it's hard to get people to write documentation ...)