Fix the example trace configure option.
Update the text to say that multiple backends are allowed and what
happens when multiple backends are enabled.

Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Message-id: 1412691161-31785-1-git-send-email-dgilbert@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>