Edits and fixes based on review.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
See also https://github.com/docker/docker/pull/8222#discussion_r18243645 for more discussion about this.

I'll copy that rationale here for clarity:
> Yeah, I agree - most readers, especially those who are new to Docker, will assume that `docker run -it official-image bash` or `docker run -it official-image bash -i` will work.  If the official images use `ENTRYPOINT` for something that's not just a wrapper script, then that no longer works, and the beginner now needs to learn about `ENTRYPOINT` and `--entrypoint`.