In `docker service create/update`, flag `--user` actually supports
`uid:gid` (same as `docker run`). However, this is not reflected
in the help and documentation yet.
This fix updates docs in `docker service create/update` to change
the description to `Username or UID (format: <name|uid>[:<group|gid>])`.
The help message output has also been updated.
This fix is related to 25304.
Signed-off-by: Yong Tang <yong.tang.github@outlook.com>