qemu-img.1: Image invalidation on qemu-img commit

qemu-img commit invalidates all images between base and top.  This
should be mentioned in the man page.

Suggested-by: Ping Li <pingl@redhat.com>
Signed-off-by: Max Reitz <mreitz@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
