In the sentence "If <tt>@post.author_ids</tt> is , this would ..." the array  is converted in a footnote, fixed this using <notextile>.
I've personally seen too many newbies get this wrong and pluralize the association
Tagged logging for which Rails has support since [3.2](http://guides.rubyonrails.org/3_2_release_notes.html#tagged-logging). It is trivial & useful but unless somebody explicitly searches API docs, he/she may not know about it. Hence, I added it in guides.
Changed the title of the 3.5 chapter from "Using the change Method" to "When to Use the change Method" since the chapter tells the migration definitions supported by the change method. This fix the duplicate ID in the document. I've also changed some of the titles to respect the Guides guidelines.
…ide [ci skip]
… [ci skip] erb tag wasn't closed well
…) [ci skip]