Because we remove doclets that are not part of the API, there
are cases where linkto will create a link to an undocumented
doclet. We now replace such links with the longname of the
doclet.
This renames the doc task to apidoc. The apidoc delegates to jsdoc3 currently. We will have additional documentation that is not built by jsdoc3. It makes sense to reserve the more general 'doc' name for building all documentation (not just apidoc).