I checked the Container Linux configuration page and found the config specification here: https://coreos.com/os/docs/latest/configuration.html (link refers to the latest version, the current version can be seen in the below field). But it was presented as a HTML list, which I thought is bad and took it upon me to create a YAML version from it for easy use.
The result can be found on GitHub Gist or simply below in the embeded Gist:
P.S. I try my best to keep it up to date. If you should notice it being outdated, please let me know by comment or email. Thanks!