doc new `app` stanza and update tests to use it

Replacing `link` for almost all cases.

The `link` stanza can still appear in error messages, because
under the hood, `app` is still implemented as a pure synonym
for the `link` artifact.  That will change automatically when
we factor into separate artifacts.
Merge pull request #6167 from rolandwalker/doc_app_stanza

doc new `app` stanza and update tests to use it