gpio: Clarify kerneldoc on gpiochip_set_chained_irqchip()

This doesn't support nested anymore, so drivers shouldn't call it with
the handler set to NULL.

Cc: Evan Green <evgreen@chromium.org>
Cc: Thierry Reding <treding@nvidia.com>
Cc: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
diff --git a/drivers/gpio/gpiolib.c b/drivers/gpio/gpiolib.c
index 6f6b4c2..17dc230 100644
--- a/drivers/gpio/gpiolib.c
+++ b/drivers/gpio/gpiolib.c
@@ -1704,8 +1704,7 @@
  * @parent_irq: the irq number corresponding to the parent IRQ for this
  * chained irqchip
  * @parent_handler: the parent interrupt handler for the accumulated IRQ
- * coming out of the gpiochip. If the interrupt is nested rather than
- * cascaded, pass NULL in this handler argument
+ * coming out of the gpiochip.
  */
 void gpiochip_set_chained_irqchip(struct gpio_chip *gpiochip,
 				  struct irq_chip *irqchip,