Polymorphic option should be either deprecated or documented better #140

It is not clear that polymorphic option takes priority over serializer option. Also seems that serializer option is recommended way for polymorphic relations: https://github.com/jsonapi-serializer/jsonapi-serializer#specifying-a-relationship-serializer

Code state: https://github.com/jsonapi-serializer/jsonapi-serializer/blob/master/lib/fast_jsonapi/relationship.rb#L74 and this behaviour is not documented at all.

My case: I used both polymorphic and serializer in one statement and for few hours could not realise why it does not call serializer block.