The MongoDB docs say my config file should be YAML. It's not. Why?
Well, the new format is YAML, but 2.6 still supports the old format:
The 2.4 configuration file format remains for backward compatibility.
You can find documentation on the old format here and as you have discovered, there are far more examples out there of the old format then on the new YAML based one. I'm not aware of anything that you can do in the new format that you can't do in the old. And I'm aware of a number of people struggling to configure the new YAML format due to few examples being available and the touchiness of the new format to spacing/tabs/etc.
Personally I'm still using the old format and will continue to do so until I see a compelling reason to switch.
Here's the docs on the old format:
http://docs.mongodb.org/v2.4/reference/configuration-options/