SPARK-1944 Document --verbose in spark-shell -h

https://issues.apache.org/jira/browse/SPARK-1944

Author: Andrew Ash <andrew@andrewash.com>

Closes #1020 from ash211/SPARK-1944 and squashes the following commits:

a831c4d [Andrew Ash] SPARK-1944 Document --verbose in spark-shell -h

(cherry picked from commit 35630c86ff0e27862c9d902887eb0a24d25867ae)
Signed-off-by: Reynold Xin <rxin@apache.org>