Make it more clear that "Public" really means "Public"

Summary: In the long term I think we can probably just explain this feature better, but for now I want to make sure no one makes a mistake with "Public". This seems like a reasonable way to do it.

Test Plan: Looked at the policy selector dropdown.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Differential Revision: https://secure.phabricator.com/D3486