Edits and fixes based on review.

Docker-DCO-1.1-Signed-off-by: Fred Lifton <fred.lifton@docker.com> (github: fredlf)
We're not talking about Docker's documentation here, which is where MkDocs
applies.

What we're talking about here is the "long description" for official
images, which gets pushed to the Hub.  The Hub doesn't fully support GitHub
Flavored Markdown, but it is their target to do so.