qapi-schema: Update description for NewImageMode

If the NewImageMode is "absolute-paths" but no backing file is available
(e.g., when mirroring a device with an unbacked image), the target image
will not be backed either. This patch updates the documentation in
qapi-schema.json accordingly.
Signed-off-by: NMax Reitz <mreitz@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>
Reviewed-by: NBenoit Canet <benoit@irqsave.net>
Signed-off-by: NKevin Wolf <kwolf@redhat.com>

qapi-schema.json

@@ -1736,7 +1736,8 @@
 # @existing: QEMU should look for an existing image file.
 #
 # @absolute-paths: QEMU should create a new image with absolute paths
-# for the backing file.
+# for the backing file. If there is no backing file available, the new
+# image will not be backed either.
 #
 # Since: 1.1
 ##