adds the ability to modify the `documentation` property inside handlers

Ran into an issue where I have a couple rest controllers using a trait.  In the trait, I have documentation for a method that I wanted to overwrite / manipulate in a custom annotation handler based on the entity the rest controller was for.

There currently is no getter for the `documentation` property and the documentation is extracted from the doc block after the handlers are called, which would overwrite anything assigned using the setter method from a handler.

This changes the order and adds a getter method.  This seems like a simple change with no BC breaks.