Note '-V 2' is necessary for auth 2.0

Here are some questions in Launchpad, just because they miss '-V 2' for
auth 2.0.
https://answers.launchpad.net/swift/+question/231938
https://answers.launchpad.net/swift/+question/231279
So I add somethins for noteing people add the '-V 2'

Change-Id: I2e0fef969be4eca316975de03fdbc516a1069acf