polymorphic type support for docs

[jasonayre]

make polymorphic types show up in the docs as "one of", with the select switching between the type attributes and what not. haven't had time to write tests but, screenshot that it's working below. also integration tested fetching and creating and what not. - works nested in polymorphic as well.

would love more documentation for attributor btw :) - I went down a bunch of different failed rabbit holes initially because I missed how to do some pretty basic attributor stuff (and the whole polymorphics.rb file in spec/support, which made me realize I could declare things inline and generic types as models)

[blanquer]

The code looks good to me, thank you. (Polymorphic types is not something that I am aware is used a lot, but it is good to have it covered).

Would you mind adding a quick, simple test for it?

[blanquer]

Also, regarding docs. It is not much, but there is a little bit of documentation and videos in the wiki part of the repo: https://github.com/praxis/attributor/wiki

It's quite old, but I believe it is both relevant, and accurate still.

[jasonayre]

@blanquer added test ^