i dont really get the docs for spl_autoload
bool spl_autoload_register ([ callback $autoload_function ] )
from my understanding, it will
Your entire statement is correct.
spl_autoload_register
allows you to register multiple autoload functions. In the case that you try to create or use an object of a class that has not been loaded into the PHP environment, PHP will run all your autoload functions looking for that class.
The Doctrine example you give is using what is called a callback to register a method inside of a class. A callback is simply an array containing the class name (for static methods) or an instance of that class (for non-static methods), and the method name.