Add the polymorphic component to the docs

[rafael-lua]

My suggestion here is to document the polymorphic component. I see it is exported, so I imagine we can use it? But since it is not documented, I'm not sure if it is only safe for internal usage.

It would be cool to use the same as logic in my custom components as well (I'm just being lazy and want to use the solution you guys already constructed).

Lovely library, thank you for the hard work.