Adding more documentation to util.inherits

I don't know if you guys will find this useful but i am pretty new to javascript and the first impression that i had from util.inherits ws that it would maintain anything that i have done with my prototype before calling it (i has some problems because of that :-).

The documentation already says the the prototype will be set to a new object, so maybe it was lack of attention (or i could imagine that the new prototype would maintain all methods already present on the constructor prototype), but now it is pretty clear the side effect of calling util.inherits on a constructor.