bug#6607: 24.0.50; Doc string fix for use-region-p

[Chong Yidong]

bojohan@gnu.org (Johan "Bockgård)" writes:

>  The return value is t provided Transient Mark mode is enabled and
>  the mark is active; and, when `use-empty-active-region' is
> -non-nil, provided the region is empty.  Otherwise, the return
> +nil, provided the region is non-empty.  Otherwise, the return
>  value is nil.

Thanks.  I checked in a different fix that better clarifies this
sentence.