3PAR driver clone volume doc updates

Adding the -online option to the operation that is called on the
3PAR backend changed what extra specs keys are supported for
cloned volumes. This patch adds a note on exactly what is supported
now. The -online option was added to resolve some performance issues

Change-Id: Idada7b2ae12b97953ed36d865f129574bb39be96
Fixes: bug 1211974