docker-storage-setup: Fix man page for DATA_SIZE documentation
docker-storage-setup: Fix man page for DATA_SIZE documentation

Improve documentation for DATA_SIZE.

Signed-off-by: Vivek Goyal <61d11262d0d9873ddb5a887f48fcb04e8c5973d0@redhat.com>

thescriptbender/container-storage-setup,shishir-a412ed/docker-storage-setup