add ability to use Grape::Entity documentation in the params block

You can use entity documentation in the params block

```ruby
class User::Entity < Grape::Entity
  expose :id,          documentation: { type: Integer}
  expose :name,        documentation: { type: String, desc: "Name" }
  expose :email,       documentation: { type: String, desc: "Email address" }
end

class Users < Grape::API
  desc 'Create user'
  params do
    requires :all, except: [:name], using: User::Entity.documentation.except(:id)
  end
  post 'users' do
    # implement create user
  end
end
```