docker-storage-setup: Update man page
docker-storage-setup: Update man page

Update man page to document new option STORAGE_DRIVER. Also edit exisiting
documentation of GROWPART which was overflowing to next line making it
look ugly.

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

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