Added and updated Doxygen comments in content handler and diff namespaces

Added and updated Doxygen comments in content handler and diff namespaces

Reasons for touching this are:
* "@param type $var" were mixed in a lot of places. Both works but the
  MediaWiki coding conventions suggest that specific order.
* Things like String and Bool aren't objects and shouldn't be uppercase.
* Tried to fill missing types in "@param $var".
* Tried to fill missing descriptions in "@return type" when I could.
* Removed duplicate descriptions if a @see is sufficend.
* Removed useless descriptions ("isUsefull returns true if usefull").
* Removed useless @return void.
* Replaces mixed[] with array (does have the exact same meaning).
* Tried to find better replacements for "varargs", phpDocumentor
  suggest $var,...
* Order should be @since, @param, @throws, @return, @see. This is the
  order Doxygen renders this.

There is always more to do but I think this is already much better
than before. Please feel free to put more change sets on top of mine
or request more changes by adding comments.

Change-Id: I05262ce06caabf79f68772302524ac91bbead1c6