builder: Update documentation WRT environment replacement

@fredlf @SvenDowideit @jamtur01 @tiborvass 
Fixed.

> On Oct 27, 2014, at 4:27 PM, Fred Lifton <notifications@github.com> wrote:
> 
> In docs/sources/reference/builder.md:
> 
> > @@ -104,6 +104,46 @@ 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
> > +
> > +**Note:** prior to 1.3, `Dockerfile` environment variables were handled
> > +similarly, in that they would be replaced as described below. However, there
> > +was no formal definition on what instructions handled environment replacement
> on what --> as to which. Missed it before, sorry.
> 
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> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/8807/files#r19444880>.
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