The first source I always check are man pages, both for the program and for the config files for that program. Then anything else is normally on the Arch or Gentoo wiki. The are very few topics that aren't covered by one or more of these sources!

I think most people don't consider checking those wikis because they aren't using those distros, but right now they are the largest and most comprehensive source of information that we have and most of the content is generic and works with any distro or even non-linux OSes.