assert_select_jquery

I'm going through the Agile Web Development with Rails book for rails 3.2, and it introduces the assert_select_jquery assertion...

The thing is, I can't find any actual official documentation for this or an explanation of how it works. Searching for "jquery" on http://api.rubyonrails.org/ turns up nothing. Is this assertion not officially supported or something?

It's in the jquery-rails gem, where all the jquery specific stuff is.

Fred.