adding support for port ranges on --expose

Addresses #1834

Signed-off-by: Srini Brahmaroutu <srbrahma@us.ibm.com>
This is the text that should be used in `docker-run.1.md`, `cli.md` and `run.md` - as the text written should look identical to what the user sees when they type `docker run ---help`.

I have a script that will update it for you (though you need to ignore some of its changes) - `docs/docs-update.py ./bundles/1.2.0-dev/binary/docker` - this will use your modified docker binary to update the documentation. :)