kbase: incrementalbackupinternals: Add snapshot terminology
kbase: incrementalbackupinternals: Add snapshot terminology

Make it obvious what's meant by 'overlay' and 'backing image' for sake
of extension of the document.

Signed-off-by: Peter Krempa <2cf5c04c61aa466e4a47bfedc747d17279c72ffc@redhat.com>
Reviewed-by: Eric Blake <a9da18479c871fe903d9590822cfe2629c6a4d11@redhat.com>

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