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

[Drew Adams]

> > 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'.

It has the effect (meaning) that, as the two comments in that
replace.el code take the trouble to say, it "resets recenter
cycling order to initial position".

With that info added to the doc string, those comments can be
removed, and Lisp users will understand immediately that they
can do the same thing: bind it to nil to get that effect.