[CI:DOCS] Explicitly mention that checkpointing systemd containers might fail
Checkpoint a container using systemd as entrypoint will probably result
in an error as CRIU cannot handle all the resources latest systemd uses.
This updates the checkpoint man page with this information.
@@ -9,6 +9,8 @@ podman\-container\-checkpoint - Checkpoints one or more running containers
## DESCRIPTION
**podman container checkpoint** checkpoints all the processes in one or more *containers*. A *container* can be restored from a checkpoint with **[podman-container-restore](podman-container-restore.1.md)**. The *container IDs* or *names* are used as input.
+*IMPORTANT: If the container is using __systemd__ as __entrypoint__ checkpointing the container might not be possible.*
+
## OPTIONS
#### **--all**, **-a**
