docs: add minor clarifications for librbd encryption
docs: add minor clarifications for librbd encryption

This should make the documentation less confusing mainly for
Ceph people.

Signed-off-by: Or Ozeri <b198acf2721cf4451c40ecf5394a1803d684f6ef@il.ibm.com>
Reviewed-by: Michal Privoznik <83d82aaba2eed257f4814b0c239c260c4caaadf0@redhat.com>

nertpinx/libvirt,olafhering/libvirt,nertpinx/libvirt,libvirt/libvirt,nertpinx/libvirt,libvirt/libvirt,crobinso/libvirt,crobinso/libvirt,nertpinx/libvirt,zippy2/libvirt,jfehlig/libvirt,crobinso/libvirt,libvirt/libvirt,olafhering/libvirt,olafhering/libvirt,zippy2/libvirt,crobinso/libvirt,jfehlig/libvirt,nertpinx/libvirt,libvirt/libvirt,zippy2/libvirt,jfehlig/libvirt,zippy2/libvirt,olafhering/libvirt,jfehlig/libvirt