PUBDEV-5196: Document categorical column support in Aggregator (#1895)
PUBDEV-5196: Document categorical column support in Aggregator (#1895)

Fixed the User Guide to indicate that categorical columns are
supported. Provided info about how exactly Aggregator handles each
categorical column.
h2oai/h2o-3,h2oai/h2o-3,michalkurka/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-3,michalkurka/h2o-3,michalkurka/h2o-3,spennihana/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,spennihana/h2o-3,h2oai/h2o-3,h2oai/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,spennihana/h2o-3,h2oai/h2o-dev,h2oai/h2o-dev,michalkurka/h2o-3,michalkurka/h2o-3,spennihana/h2o-3,h2oai/h2o-3,h2oai/h2o-3,h2oai/h2o-dev