Make hidden and locked configuration even more explicit

Summary:
A user in IRC seemed very confused by this, and worked extremely hard to shoot themsevles in the foot by manually writing locked configuration to the database.

Try to explain why configuration is locked better.

Test Plan:
Mostly reading.

{F1078905}

{F1078906}

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15128