Use trigger type 15 to indicate disabled. (#721)

* Add disabled trigger type.

Also clarify tdata2/tdata3 behavior when a trigger is disabled.
Add a paragraph about context restoring triggers.

* Change trigger type 15 to mean disabled.

Nobody can remember why a trigger might be unavailable, or imagine what
that even means.

* Formatting

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>