Documented changes to API to enable new `docker cp` behavior.
Added documentation on `docker cp` usage and behavior.
Docker-DCO-1.1-Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
Upstream-commit: e54b1e081ae15a71e758f7d2691dc2bff3f66e02
Component: engine