force run container detached if container CreateCommand missing the detach param

the podman generated systemd service file has `Type=forking` service,
so the command after `ExecStart=` should not run in front.
if someone created a container and has the detach(`-d`) param missing
like this
```
podman create --name ngxdemo -P nginxdemos/hello
```
and generate the file with `--new` param:
```
podman generate systemd --name --new ngxdemo
```
because `podman run xxx` has no `-d` param,
so the container is not run in background and nerver exit.
and systemd will fail to start the service:
```
sudo systemctl start container-ngxdemo.service
Job for container-ngxdemo.service failed because a timeout was exceeded.
See "systemctl status container-ngxdemo.service" and "journalctl -xe" for details.
```

Signed-off-by: 荒野無燈 <ttys3@outlook.com>

1 files changed, 1 insertions, 0 deletions

diff --git a/docs/source/markdown/podman-generate-systemd.1.md b/docs/source/markdown/podman-generate-systemd.1.md
index 4d3f9ba48..3199232fa 100644
--- a/docs/source/markdown/podman-generate-systemd.1.md
+++ b/docs/source/markdown/podman-generate-systemd.1.md
@@ -25,6 +25,7 @@ Use the name of the container for the start, stop, and description in the unit f
 **--new**
 
 Create a new container via podman-run instead of starting an existing one.  This option relies on container configuration files, which may not map directly to podman CLI flags; please review the generated output carefully before placing in production.
+Since we use systemd `Type=forking` service, using this option will force the container run with the detached param `-d`
 
 **--timeout**, **-t**=*value*