DOC: update text classification docs for CompositeFieldAggregator

See TXTREC-72:

Currently, we have hardcoded field definitions in the PageAggregator but we should provide our users with an easy way to write some logic to achieve aggregation with their custom fields.

AC

• Have an API to extract one field per class e.g. CompositeFieldAggregator
• Each aggregator knows about the type(s) they can handle
• Aggregators are multibinded and traversed over until one aggregator feels responsible to respond.