bug#3598: 23.0.94; doc string of frame-root-window

[Drew Adams]

> > If it is an internal function, then please either do not 
> > give it a doc string or, preferably, give it one that calls
> > that out explicitly, and please add a
> > comment explaining why it is internal.
> 
> I see no reason not to have doc strings for internal functions.
> Emacs hackers need doc strings from time to time.  Adding "internal
> use only" to the doc string is the preferred solution.

FWIW, I agree 100%.