Add documentation about Mapper capabilities

I've been asked a lot if an endpoint could return `Future` or `A` or `B`.
Currently, it states that one could consider an arity and apply function to Endpoint: https://github.com/finagle/finch/blob/master/docs/src/main/tut/user-guide.md#mapping-endpoints

The example is slightly incorrect, also it's worth to mention all possible types this function can return. On a side note, it could be useful to add an _@implicitNotFound_ message to Mapper(?) and list all these types there also

Start in docs/src/main/tut/user-guide.md at the Mapping Endpoints section and inspect Mapper to identify the supported return types. Correct the example and document the possible return types; assess separately whether an @implicitNotFound message belongs in scope. Done means the guide accurately reflects Mapper capabilities.

Written by the indexing model from the issue text.