[Fixes #11734]Correct Association Basics Guide on has_and_belongs_to_many

Done the following changes:
 1. Replace `uniq`  in example with `autosave` as `uniq` is not supported
 on `has_and_belongs_to_many`
 2. Add `:readonly` option to list of supported call as per [EdgeApi](http://edgeapi.rubyonrails.org/classes/ActiveRecord/Associations/ClassMethods.html#method-i-has_and_belongs_to_many)