Add comment to settings.yml to nudge admins towards editing values via Web UI (#1289)

Add comment to settings.yml to nudge admins towards editing values via Web UI (#1289)

* Put a useful message for new admins on /about/more

I totally failed to realize this file was just defaults.  I think
this message would be a good default for people like me.

* Revert default site description, expand comment

This will keep setup-related stuff from leaking into public views,
while still hopefully keeping over-eager admins from editing this
file unnecessarily before RTFMing.  (e.g., me)