Update docs in docker service create/update for flag --user

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>
Upstream-commit: aee09a390b
Component: cli

components/cli/docs/reference/commandline/service_create.md

@@ -42,7 +42,7 @@ Options:
       --update-delay duration          Delay between updates
       --update-failure-action string   Action on update failure (pause|continue) (default "pause")
       --update-parallelism uint        Maximum number of tasks updated simultaneously (0 to update all at once) (default 1)
-  -u, --user string                    Username or UID
+  -u, --user string                    Username or UID (format: <name|uid>[:<group|gid>])
       --with-registry-auth             Send registry authentication details to Swarm agents
   -w, --workdir string                 Working directory inside the container
 ```

components/cli/docs/reference/commandline/service_update.md

@@ -50,7 +50,7 @@ Options:
       --update-delay duration          Delay between updates
       --update-failure-action string   Action on update failure (pause|continue) (default "pause")
       --update-parallelism uint        Maximum number of tasks updated simultaneously (0 to update all at once) (default 1)
-  -u, --user string                    Username or UID
+  -u, --user string                    Username or UID (format: <name|uid>[:<group|gid>])
       --with-registry-auth             Send registry authentication details to Swarm agents
   -w, --workdir string                 Working directory inside the container
 ```