bug#10539: 24.0.92; low display-time-interval makes Emacs slow

[Drew Adams]

> > IOW, (force-mode-line-update t) is a way to force the next redisplay
> > cycle do a more thorough job, it has nothing to do with mode lines per
> > se.
> 
> I think it would be good if you could document this, because it's
> basically impossible to figure out what this function 
> actually does from looking at its lisp definition.

+1