title: CanCan explanation of load_and_authorize_resource
body: <p>I would know how the <code>load_and_authorize_resource</code> works inside.
  I searched the github page <a href="https://github.com/ryanb/cancan/blob/ff2b632f1524bac4273fe113968d73ea62114652/lib/cancan/controller_additions.rb#L357" rel="noreferrer">Link</a> and tried to undestand , but i didn't find nothing usefull. I only understand that <code>load_and_authorize_resource</code> is like a before_filter and it loads (in some way) the ability that we have written in ability.rb</p>
  
  <p>I would know better how this is possible. I mean, i don't want to study ALL the gem, but i want just to see how cancan load the ability of a resource in a controller and if the <code>load_and_authorize_resource</code> is really a sort of before_filter.</p>