docs/system: Update documentation for s390x IPL

Update docs to show that s390x PC BIOS can support more than one boot device.

Signed-off-by: Jared Rossi <jrossi@linux.ibm.com>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-ID: <20241020012953.1380075-19-jrossi@linux.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>
2 files changed