@Jacquob 

If the detailed guides regarding Parental Controls or other features are included into the devices's user guide, it will grow into a very big document that would be hard to read.

Probably that's the reason the detailed guides regarding a particular functionality to be separated in a FAQ documents, like the one you mentioned.

The other reason could be if they are separated from the user guide, they are easier to update / modify (when a new features are implemented) than if they were a part of the device's user guide.