The `cross_fields` type is particularly useful with documents where the fields 
are logically dependent.  For instance a document with a `first_name` and
`last_name` field can produce skewed scores if a given term (like `Smith`) could
be an uncommon `first_name` as well as a common `last_name`. Due to the nature of TF/IDF
a query for `Smith` would push the document with `Smith` as a first name to the top of the results list.
The `cross_fields` type tries to treat fields `statistically` as if they were a single field, in order to
provide more intuitive scoring and more relevant results.