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)

config/settings.yml

 # config/app.yml for rails-settings-cached
+#
+# This file contains default values, and does not need to be edited
+# when configuring an instance.  These settings may be changed by an
+# Administrator using the Web UI.
+#
+# For more information, see docs/Running-Mastodon/Administration-guide.md
+#
 defaults: &defaults
   site_title: 'Mastodon'
   site_description: ''