Converted Doxygen to PHPDoc in eZContentObject

Working with IDEs like PHPStorm gets a lot more convenient with correct PHPDocs - following #643, I converted the Doxygen to PHPDoc in `eZContentObject`:

* The descriptions have mostly been copied 1:1
* Where no descriptions have been present, I tried to add a small one
* **No other code changes have been made**

Cheers
 Jérôme