Updating Associations::Preloader docs

Much of the previous documentation introduced features new in 2011. This
commit refreshes it to provide clearer code examples and spends more
time describing the normal case (preloaded associations) and less time
describing the fallback.

[ci skip]
This example is incorrect. This query would never be generated by Active Record because it doesn't include a WHERE clause that span across the tables. I believe it is better to use a real example.