Rewrite ENTRYPOINT documentation covering all the combinations with exam...

...ples.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)

@fredlf @jamtur01 @metalivedev

This PR documents how the ENTRYPOINT instruction works today.

I got confused here. I'm not sure what "it" is referring to. The entrypoint, the shell, something else?