Add docs for accessing behaviors.

Refs cakephp/cakephp#5264
Should we also add a note that using plugin behaviors (and other objects), you may not use the plugin syntax for has/get?

    $table->addBehavior('MyPlugin.CounterCache');

but

    $table->behaviors()->has('CounterCache');
    $table->behaviors()->get('CounterCache');

May not be obvious to most.
The alternative is to allow both syntaxes for has/get, as well.