Summary:
The only purpose of "Arcanist Overview" is to tell people they shouldn't be here, but we bury the lede.
Make it clear that this is not user documentation.
After T988 we can improve the organization here, but some recent users found this pretty confusing.
Test Plan: Generated and read documentation.
Reviewers: vrana, btrahan
Reviewed By: vrana
CC: aran
Differential Revision: https://secure.phabricator.com/D3235
