Modify the help message of `-c`/`--column` parameter
Modify the help message of `-c`/`--column` parameter

Some users ues `-c`/`--column` parameter in the wrong way, such as:
--column='ID Name'
--column='ID:Name'
--column='ID,Name'
The patch modifies the help message of `-c`/`--column` parameter
to indicate the correct use of the parameter.

Change-Id: I7a5082ea6cbb91e2c5d3348f5a52558aab09d820
Task: 29052
Story: 2004845

openstack/cliff,openstack/cliff