virDomainCheckpointCreateXML: Add disclaimer about creating checkpoints
virDomainCheckpointCreateXML: Add disclaimer about creating checkpoints

Checkpoints created via virDomainCheckpointCreateXML are generally not
very useful as they need to be coupled with a backup.

Add a disclaimer to the docs explaining why users should use
virDomainBackupBegin instead.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Jonathon Jongsma <c7254805a17bd5c41e24c3a278fcbf3afcdfb0fa@redhat.com>

libvirt/libvirt,zippy2/libvirt,olafhering/libvirt,libvirt/libvirt,olafhering/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,zippy2/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt,jfehlig/libvirt,libvirt/libvirt,libvirt/libvirt,jfehlig/libvirt