Adding developer notes about nested irqs

lib/LinkWireless.cpp

@@ -32,4 +32,19 @@ LINK_CODE_IWRAM void LinkWireless::_onTimer() {
 #endif
 }
 
+/**
+ * NOTES:
+ * When using `LINK_WIRELESS_ENABLE_NESTED_IRQ`:
+ *   - Any user ISR can interrupt the library ISRs.
+ *   - SERIAL ISR can interrupt TIMER ISR.
+ *     -> This doesn't cause data races since TIMER ISR only works when
+ *        there is no active async task.
+ *     -> When TIMER ISR starts an async task (`transferAsync(...)`),
+ *        nested interrupts are disabled (`REG_IME = 0`) and SERIAL cannot
+ *        interrupt anymore.
+ *   - TIMER interrupts are skipped if SERIAL ISR is running.
+ *   - VBLANK interrupts are postponed if SERIAL or TIMER ISRs are running.
+ *   - Nobody can interrupt VBLANK ISR.
+ */
+
 #endif