provide example for has_many, order: title.asc

[westonplatter]

using:

• mongoid 4.0.0.beta
• rails 4.0.2

making a note and tagging myself (@westonplatter), so I remember to come back to this.

the docs don't current describe that the order key-value pair should have the sort order (either asc or desc) tacked onto the sort attribute. therefore, the docs should have an example like this:

class SomeModel 
  include Mongoid::Document
  has_many :other_models, order: :attribute.asc
end