builder: Update documentation WRT environment replacement

@fredlf @SvenDowideit @jamtur01 @tiborvass 
OK. You guys need to figure this out — which style is best -- and get back to me.

> On Oct 27, 2014, at 3:30 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)
> Not sure we want or need this version information. The docs are versioned, so we should just describe the behavior in the current version. If we want to to broadcast the change, I'd suggest we use release notes for that.
> 
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