docs: Document managedsave-edit commands support
    
    This patch documents support for managedsave-dumpxml,
    managedsave-define and managedsave-edit commands.
    
    Signed-off-by: Kothapally Madhu Pavan <kmp@linux.vnet.ibm.com>
    Reviewed-by: Andrea Bolognani <abologna@redhat.com>