Fix config-migration text for dashboard options

Summary:
The commit which added checks for the old homepage options (now in
Dashboard) in rP9d9a47e9cf, added them to the auth section, where they
would present:

This option has been migrated to the "Auth" application. Your old
configuration is still in effect, but now stored in "Auth" instead of
configuration. Going forward, you can manage authentication from the
web UI.

Remove them from the moved-to-Auth list, and coalesce the multiple
definitions of the help text into one.

Test Plan:

• set maniphest.priorities.unbreak-now to something
• observe the setup issue reported
• hope it tells you the right thing

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: Korvin, epriestley, chad

Differential Revision: https://secure.phabricator.com/D16576