I was thinking we'd keep ithread.9, but maybe it doesn't really make sense to do so.

Is "entr" a typo for "intr"?

Since you're touching that area, "an interrupt thread" should be "the interrupt thread" here, if you're referring to the same one mentioned on the previous line.

Reminder (if needed) to bump on commit.

While there, maybe change this to .Xr swi_add 9 to make it clearer it's not just an example and for easier comprehension of what the example does, not just how it does it? (I'm not part of the intended audience, so if you feel it's not needed, that's fine with me.)

Maybe .Xr intr_event 9?

Yes, will do.

Sure, it will be done as part of D33476.

I wanted to retain the reference, but I did not find a way to word it that makes sense. "intr_event" is the new name of the KPI, but you wouldn't really refer to an object as an "intr_event" in the same way that you can with an "ithread".