bug#29328: 24.5; doc string of `transpose-subr`

[Eli Zaretskii]

> From: Noam Postavsky <npostavs@users.sourceforge.net>
> Date: Fri, 17 Nov 2017 10:23:24 -0500
> Cc: Eli Zaretskii <eliz@gnu.org>, 29328@debbugs.gnu.org
> 
>    "Subroutine to do the work of transposing objects.
>  Works for lines, sentences, paragraphs, etc.  MOVER is a function that
>  moves forward by units of the given object (e.g. forward-sentence,
> -forward-paragraph).  If ARG is zero, exchanges the current object
> +forward-paragraph).  If SPECIAL is non-nil, then MOVER should
> +return the bounds of the object as a cons (BEG . END) instead.

It's the other way around: SPECIAL doesn't require MOVER to do
anything, it's an INDICATION that MOVER does something "special".

Please just drop this.  It's another bikeshedding argument.  We don't
need to document this.