Describe full behaviour of Active Record's attribute query methods

Update the docs to more accurately describe how the Active Record attribute query methods behave ([applicable code here](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/attribute_methods/query.rb)).
Also, I don't think it's worth calling out anything in this branch of code: https://github.com/rails/rails/blob/master/activerecord/lib/active_record/attribute_methods/query.rb#L17-L24, as it will never be reached under normal circumstances.