[RF][Docs] Improve docs of RooAbsArg.
[RF][Docs] Improve docs of RooAbsArg.

olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root,olifre/root