ChangeSecret needs better documentation

Right now, the only mention of how to change an instance secret is a note on the instance.secret property and the example invocation is wrong.

The correct invocation:

accumulo org.apache.accumulo.server.util.ChangeSecret

The instructions should specify the following caveats

1. The cluster must be down before running (verified by the tool)
2. The command should be run as the user that controls the existing Accumlo files in HDFS (ref ACCUMULO-2971)

The instructions for use should probably go in the troubleshooting section of the manual, and then get a link from the instance.secret property as well as the accumulo settings section that talks about the instance secret.