IMPALA-931: Indicate in impala-shell --help that verbose output is enabled by default

Change-Id: I6c29d542c4379d46fcdce62905c89b319628dd61
Reviewed-on: http://gerrit.ent.cloudera.com:8080/2842
Reviewed-by: Abdullah Yousufi <abdullah.yousufi@cloudera.com>
Tested-by: jenkins