Explain what the VIRTUAL SIZE means

Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au>

Docker-DCO-1.1-Signed-off-by: Sven Dowideit <SvenDowideit@home.org.au> (github: SvenDowideit)

docs/sources/reference/commandline/cli.md

@@ -796,6 +796,10 @@ decrease disk usage, and speed up `docker build` by
 allowing each step to be cached. These intermediate layers are not shown
 by default.
 
+The `VIRTUAL SIZE` is the cumulative space taken up by the image and all
+its parent images. This is also the disk space used by the contents of the
+Tar file created when you `docker save` an image.
+
 An image will be listed more than once if it has multiple repository names
 or tags. This single image (identifiable by its matching `IMAGE ID`)
 uses up the `VIRTUAL SIZE` listed only once.