Document enums

[mxmCherry]

Maybe to be used like this:

expose :enum_one, documentation: { ..., values: [1, 2, 3] }
expose :enum_two, documentation: { ..., values: Model.enum_twos.keys }

[mxmCherry]

#21 (comment)

Most likely, both entity fields and route params could use the same 4-column table (with Values col): https://github.com/ruby-grape/grape#values

To be addressed as part of this issue, but later.