Cleanup: correct cli --volumes-from description

issue #7580 volumes-from comma separated list mentioned this case.

Options with [] may be specified multiple times. it does mean docker can
support comma separated list.
Example:
case 1, works:
docker run -it --rm --volumes-from TEST_DATA --volumes-from TEST_DATA2 ubuntu
bash

case 2, failed:
docker run -it --rm --volumes-from TEST_DATA,TEST_DATA2 ubuntu bash
2014/08/20 15:40:35 Error response from daemon: Cannot start container
e2f63ce5611c7a1be28389580d4a3847713078e6d5b61aac6cd532ec68b02191: Container
TEST_DATA,TEST_DATA2 not found. Impossible to mount its volumes

Signed-off-by: Deshi Xiao <dxiao@redhat.com>