(DOCUMENT-22) Clarify the cron properties user and target

For the default cron provider, `user` and `target` are tightly coupled which
creates a brittle and unhealthy relationship. Re-worded the doc strings for
these properties to call out this relationship and recommend that `user` be
used instead of `target`.
(maint) Document Cron autorequre of user resources

A Cron resource will autorequire the an User resource with the name specified
by its `user` property.