Add basic documentation for the AttributeBundle

Q  | A
----|----
Doc fix? | no
New docs? | yes
Fixed tickets | 

Interestingly whilst documenting the bundle configuration, I noticed that the `validation_groups` where named `property` and `product_property`.

Aside from the fact that the bundle should not aware of the concept of a `product`, am I right in thinking that `property` was the old name for the attribute?