updated documentation for mentioning images vs containers

Docker-DCO-1.1-Signed-off-by: Robin Speekenbrink <robin@kingsquare.nl> (github: fruitl00p)

rebased by

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)
Merge pull request #6099 from SvenDowideit/pr_out_updated_documentation_for_mentioning_images_vs_containers

updated documentation for mentioning images vs containers