Converted Doxygen to PHPDoc in eZDataType and all derived datatypes

* The most relevant changes have been made in `eZDataType`
* All other datatypes inherit the PHPDocs automatically, that's why the old doxygen docs have been removed
* Other descriptions have mostly been copied 1:1
* Where no descriptions have been present, I tried to add a small one or kept it empty except the `@param` and `@return` definitions

**No other code changes have been made**

Cheers
:octocat: Jérôme