Clarify that icount decrements on interrupt traps.

The rationale is that if icount is enabled in the current mode and the
mode of the trap handler, when you single step you don't want to step
past the first instruction of the trap handler, which you didn't know
was going to be executed. Having to step multiple times to get past an
interrupt to actually execute the instruction you tended to step is
addressed the same way as already documented in the appendix.

Addresses a question from #842.