[Bug] Small documentation inconsistencies

[nywilken]

• When reviewing the documentation I found that there is no information around the contents of the network_interfaces block. The examples are helpful, but for documentation purposes it could help by having a small section on the values of that make up the network_interfaces array.
• template_prefix indicates that it defaults to an empty string but the resulting image has a prefix of custom-image. That should be in the documentation.

[thevilledev]

Thank you @nywilken, both fixed in #22!