better snapshot create help message
better snapshot create help message

Signed-off-by: Peng Tao <39d8af57313740070ea49547576f55682fe5221a@gmail.com>

Jimmy-Xu/hypercli,imeoer/hypercli,hyperhq/hypercli,imeoer/hypercli,hyperhq/hypercli,Jimmy-Xu/hypercli,hyperhq/hypercli,imeoer/hypercli,Jimmy-Xu/hypercli