manpage: virsh: Note that 'virsh undefine --remove-all-storage' doesn't remove backin...

manpage: virsh: Note that 'virsh undefine --remove-all-storage' doesn't remove backing images

Removing a backing image could break other image chains as it's
theoretically possible to share backing chains.

As --storage/--remove-all-storage is fully implemented in virsh as a
helper option, which enumerates and deletes VM's volumes. We do not plan
to make it any more complicated.

Document that backing chains are not removed.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>