bug#33331: 26; doc strings of `tabify' and `untabify'

[Eli Zaretskii]

> Date: Sat, 10 Nov 2018 11:36:19 -0800 (PST)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 33331@debbugs.gnu.org
> 
> > If we are going to change those functions, I'd prefer instead to make
> > a change that used the 3rd argument in non-interactive calls.
> 
> I was going to say, "Sure, go for it".  But what would
> that mean in this case?  Non-interactively you already
> supply the START and END.  What would be gained by
> a Boolean argument that uses `point-min' and `point-max'
> and ignores the values of START and END?

Just consistency with the interactive use.  Not a big deal, obviously,
but then neither is the fact ARG goes on unused.