Converted Doxygen to PHPDoc in eZContentObjectTreeNode

Working with IDEs like PHPStorm gets a lot more convenient with correct PHPDocs - as the `eZContentObjectTreeNode` class is one of the most used classes, I gave it a shot and converted all Doxygen docs to PHPDoc. 

* The descriptions have been copied 1:1
* Where no descriptions have been present, I tried to add a small one
* I removed the class description because it seemed unfinished (and I didn't understand it :) )
* **No other code changes have been made**

I hope this change makes it, I would like to add PHPDoc to other classes, too.

Cheers
:octocat: Jérôme