`cross_fields` attemps to treat fields with the same analysis
configuration as a single field and uses maximum score promotion or
combination of the scores based depending on the `use_dis_max` setting.
By default scores are combined. `cross_fields` can also search across
fields of hetrogenous types for instance if numbers can be part of
the query it makes sense to search also on numeric fields if an analyzer
is provided in the reqeust.
Relates to #2959
Simon Willnauer
Clinton Gormley