trace-events: Consistently point to docs/devel/tracing.txt

trace-events: Consistently point to docs/devel/tracing.txt

Almost all trace-events point to docs/devel/tracing.txt in a comment
right at the beginning.  Touch up the ones that don't.

[Updated with Markus' new commit description wording.
--Stefan]

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20190314180929.27722-2-armbru@redhat.com
Message-Id: <20190314180929.27722-2-armbru@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>