Update Header to check field description

Problem/Motivation

For a first time user who's not a security guru, but who is trying to set this up, the Header to check field doesn't indicate a recommended header to use. I've seen a few listed in the issue queue, but it would be nice to suggest the most common ones with a note that others may be more appropriate depending on your situation.

At a minimum, suggesting that a user can start with REMOTE_ADDR may be helpful.