Separate add share group description in other API or CLI echo in
this patch from add share group itself description in other patches:
Added share group iteslf in api-ref [1]
Added share api admin doc [2]
Added share group in driver requirements [3]
[1] I852d264eb120027aad32997dc64a6ca428ee55a4
[2] Ia0cc534e0bfb2ca5e495e575237e9911c746691b
[3] Iedde5a9a774a60e760b47d5eb2973f42d79227d4
Change-Id: Iedde5a9a774a60e760b47d5eb2973f42d79127d3