Updated the description of run -d

The goal is to make it more clear this will give you the container id after run completes.

Since stdout is now standard on run, "docker run -d" is the best (or only) way to get the container ID returned from docker after a plain run, but the description (help) does not hint any such thing.
Merge pull request #1339 from dhrp/docker-run-d-description

Updated the description of run -d