AR: Clarify what tmexttrigger.intctl is for (#898)

* AR: Clarify what tmexttrigger.intctl is for

Addresses #891.

* Add missing article

Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>
Signed-off-by: Tim Newsome <tim@sifive.com>

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Signed-off-by: Tim Newsome <tim@sifive.com>
Co-authored-by: Paul Donahue <48959409+pdonahue-ventana@users.noreply.github.com>