Rewrite ENTRYPOINT documentation covering all the combinations with examples.

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@docker.com> (github: SvenDowideit)
Merge pull request #8695 from SvenDowideit/add-details-to-entrypoint-docs

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