bug#20419: 25.0.50; doc string of `recenter-last-op`

[Eli Zaretskii]

> Date: Sat, 25 Apr 2015 07:29:20 -0700 (PDT)
> From: Drew Adams <drew.adams@oracle.com>
> Cc: 20419@debbugs.gnu.org
> 
> > > The doc string says that these are the possible values:
> > > "`top', `middle', `bottom', integer or float numbers."
> > > It should also mention `nil', say what `nil' means, and
> > > perhaps even say that `nil' is the default value.
> > 
> > How do you see that nil has any meaning whatsoever?
> 
> `nil' means that the next `recenter-top-bottom' starts with
> the first element of `recenter-positions'.

Yuck!  Why not set it to the first member of recenter-positions to
begin with?

FWIW, I consider this, including the exposure of this variable as some
kind of knob for other Lisp packages, bad style.  You should be
demanding cleaning this up instead of documenting it.

Fixed under protest.