builder: Update documentation WRT environment replacement

@fredlf @SvenDowideit @jamtur01 @tiborvass 
similarly to how they were handled in prior to 1.3?

> On Oct 27, 2014, at 3:25 PM, Fred Lifton <notifications@github.com> wrote:
> 
> In docs/sources/reference/builder.md:
> 
> > @@ -104,6 +104,37 @@ be treated as an argument. This allows statements like:
> >  Here is the set of instructions you can use in a `Dockerfile` for building
> >  images.
> >  
> > +### Environment Replacement (as of 1.3)
> > +
> > +**Note:** prior to 1.3, `Dockerfile` environment variables were handled
> > +similarly, but not exactly the same -- there was no formal definition at the
> It's not at all clear to me what env. vars are being compared to here. Similarly to what?
> 
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