drivers: mcp23xxx: explain more common causes for spurious interrupts

Interrupt handling in this chip is broken beyond repair, anyone unfortunate
enough to have to use it will probably come across this error and wonder
what's up.

Signed-off-by: Armin Brauns <armin.brauns@embedded-solutions.at>

Author: arbrauns

drivers/gpio/gpio_mcp23xxx.c

@@ -412,8 +412,13 @@ static void mcp23xxx_work_handler(struct k_work *work)
 	}
 
 	if (!intf) {
-		/* Probable cause: REG_GPIO was read from somewhere else before the interrupt
-		 * handler had a chance to run
+		/* Probable causes:
+		 * - REG_GPIO was read from somewhere else before the interrupt handler had a chance
+		 *   to run
+		 * - Even though the datasheet says differently, reading INTCAP while a level
+		 *   interrupt is active briefly (~2ns) causes the interrupt line to go high and
+		 *   low again. This causes a second ISR to be scheduled, which then won't
+		 *   find any active interrupts if the callback has disabled the level interrupt.
 		 */
 		LOG_ERR("Spurious interrupt");
 		goto fail;