"downloadNote" function. ENNoteRef input, ENNote output.
Philosophically, ENNotes are still containers for content, and don't really represent a
note on the service. If you download one and then reupload/replace, you're replacing the
service note with a whole new thing. (*mostly. We will preserve some "hidden" characteristics
of the original note on update, like existing tags and reminder dates, etc. I'm not sure
this is really ideal.)