Make special notes about and document behavior of the cookbook attribute

[martinisoft]

The cookbook attribute found in the template and cookbook_file should have some more specific notes in their respective resource documents to talk about how Chef resolves the location of a source file.

The Custom Resources example section should also make special note of this since the template resource is used. I think it should talk about if you use the website resource in another cookbook that it will have to define the cookbook attribute and maybe provide an example.