Okay, I’m going to warn you: this is a deep rabbit hole and you basically have no choice but to jump in.
The best source I know of is Git-SCM. Partway down that very long page is a very long list of possible variables for the config file. Variable namespace syntax is potentially confusing here, so just remember that the words in
 in the git config file are the same as the words before the
. in names such as
GitHub is a code storage service built on the git software created by Linus Torvalds. All git repos will obey the same configuration file regardless of their hosting service, and config variables that are implemented in the current released version of git should apply across everything. That’s part of what makes it appealing as a backup and collaboration tool. It’s only fairly recently that most OSS people moved onto GitHub and made it feel synonymous with git. Because of this, GitHub help is great when you’re working specifically with GitHub, but it doesn’t have as much detail when it comes to things that you can’t do through the GitHub GUI.