Clearer help texts in admin

[stefanfoulis]

It should be possible to configure a flag without extensive studying of the docs.

e.g.

• that everyone needs to be unknown for testing to work.
• ...

[jsocol]

Flag interactions are complex and while both the in-admin help and the documentation need to be better, I don't think it's every going to be really possible to explain it without going to the docs—at least without putting a book chapter into the admin.

I'm going to take a crack at improving both for the next milestone. But it does already say in the admin that Everyone "overrid[es] all other settings". I can make that bold but there's not a lot else I can do to call it out.