Print possible options for `changes` command in help message.

Now in help for changes command we can see something like this:

   [--sort {status,topic,url,owner,project,branch,subject,createdOn,lastUpdated,approvals}]

  --sort {status,topic,url,owner,project,branch,subject,createdOn,lastUpdated,approvals}
                        Set the sort field

If this is acceptable solution I can add it for other commands