docs: childView section for CompositeView

The childView info was hiding inside the recursive by default section. It was described in the negative: "if you do not want the recursive feature, specify a childView." 
Far clearer is specifying this seperately in a positive sense in it's own section: "use a childView to render each item views template.". Also, I think people will be looking for this basic info as one of the first places they come to in Marionette (like me :)