Allows you to specify the model association key in a belongs_to
relationship instead of the foreign key.

The following queries are now equivalent:

Post.where(:author_id => Author.first)
Post.where(:author => Author.first)

PriceEstimate.where(:estimate_of_type => 'Treasure', :estimate_of_id => treasure)
PriceEstimate.where(:estimate_of => treasure)

In response to this rails issue: https://github.com/rails/rails/issues/3729

In the sentence "If <tt>@post.author_ids</tt> is [1], this would ..."
the array [1] 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.

erb tag wasn't closed well

There are two changes:
- Using <ruby> syntax for unicode character routes example instead of
  <tt> since it causes single quotes to be transformed into inverted
  commas. This makes this required change more eye-catching.
- Using <tt> for the j alias and related helpers as written in the
  documentation guidelines.

In chapter 2.4 there was an underscore at the start of the attr_readonly method name,
this underscore was interpreted as the opening of the <em> tag but it wasn't closed.

Worst, the underscore in self.class.serialized_attributes (end of paragraph)
was interpreted as the closing of the <em> tag and not as the name of a method.