bug#6000: describe-text-sexp does not know window-width

[Juri Linkov <juri@jurta.org>]

> In describe-text-sexp there is a call to window-width. I believe this
> gives unreliable results because the help-window might not be select
> at that point always.
>
> This shows up in the display of (what-cursor-position t) where the
> sexp are sometimes unnecessary hidden behind "[Show]".
>
> A possible good enough cure is to surround the call to window-width
> with something like this
>
>    (with-selected-window (or (get-buffer-window "*Help*") (selected-window))
>      (window-width))

Martin referred to this bug report from bug#17831, and I noticed
that instead of hard-coding the "*Help*" buffer name, better would be
to use (current-buffer) because the formatted buffer is current:

  (- (if (window-live-p (get-buffer-window (current-buffer) t))
         (with-selected-window (get-buffer-window (current-buffer) t)
           (window-width))
       (window-width))
     (current-column))