Added note in manpages to clarify effect of docker start on started container

When run on a container that is already running, docker always succeeds.
This was an intentional change in v0.10; it previously caused an error.
This commit clarifies the man pages to indicate this is intended behavior.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)
Clarify effect of docker start on started container in CLI docs

This behavior changed from v0.9 to v0.10, so document it to prevent
any confusion.

Docker-DCO-1.1-Signed-off-by: Matthew Heon <mheon@redhat.com> (github: mheon)