Freehackers once had some pages about gentoo flags: how to set them, which one to choose, and so on. By the time, this information was not provided by gentoo, although lot of people were googling for this. I was one of them.
Nowadays, this kind of information, up-to-date, accurate, and maintained, can be found on the official site
Some more information is provided by gentoo-wiki.com, which is an unofficial site with interesting stuff about gentoo: