builder: Update documentation WRT environment replacement

@fredlf @SvenDowideit @jamtur01 @tiborvass 
Kind of; it’s 1.3.x for *this* specific behavior, but it was similar before.

I can annotate it.

> On Oct 27, 2014, at 1:46 PM, James Turnbull <notifications@github.com> wrote:
> 
> In docs/sources/reference/builder.md:
> 
> > @@ -104,6 +104,33 @@ 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
> Is this from a specific release onwards? Is it worth calling that out?
> 
> —
> Reply to this email directly or view it on GitHub <https://github.com/docker/docker/pull/8807/files#r19435875>.
>