doc: kernel: Clarify relationship between direct and ZLI interrupts

The documentation did not state clearly that any zero-latency IRQ must
also be declared as a direct ISR. This is critical because failure to do
so may cause race conditions between the ZLI and regular ISRs when
executing the preable/postamble code in regular interrupts.

Signed-off-by: Carles Cufi <carles.cufi@nordicsemi.no>