How to define a custom example for a parameter?

Like definining ToSample in servant-docs. (Actually, I'm not sure what is the relation between servant-swagger and servant-docs, but defining ToSample won't change the default example in servant-swagger)

Start by comparing how custom ToSample definitions work in servant-docs with the current behavior of servant-swagger. Clarify how the two projects should relate for parameter examples and identify the relevant example-generation entry point. Done means a custom ToSample definition changes the default example produced by servant-swagger.

Written by the indexing model from the issue text.