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)

contrib/man/md/docker-start.1.md

@@ -20,6 +20,10 @@ the process
 **-i**, **--interactive**=*true*|*false*
    When true attach to container's stdin
 
+# NOTES
+If run on a started container, start takes no action and succeeds
+unconditionally.
+
 # HISTORY
 April 2014, Originally compiled by William Henry (whenry at redhat dot com)
 based on docker.io source material and internal work.