Added documentation support to parameters

Additional documentation can be added to a parameter by providing a `documentation` hash.

```ruby
params do
  optional :first_name, type: String, documentation: { example: 'Jim' }
  requires :last_name, type: String, documentation: { example: 'Smith' }
end
```

Previous discussion in [this issue](https://github.com/intridea/grape/issues/571).