[m-rev.] for post-commit review: fix io section of transition guide

[Julien Fischer]

Hi Zoltan,

On Wed, 11 Feb 2015, Zoltan Somogyi wrote:

> For post-commit review by anyone.
>
> I will look at the other sections later.

One other thing I noticed about the transition guide a few weeks back is
that it doesn't mention the existence of the standard libary's 'prolog'
module.

Cheers,
Julien.