Fix the documentation about link's transform attribute.

[tym-czasowy]

A little fix at docs.

[lovelydinosaur]

We're going to drop transform completely with 3.0.

coreapi originally started as an attempt to address hypermedia, so we had documents that supported including both data, links and nested documents (the nested documents is where transform came in) However the big-win utility for coreapi is really for using it with API schemas, so going to scope down the project to focus on that one case.