MAINTAINERS: add Documentation/iio/ to IIO subsystem

Patches touching the IIO subsystem documentation should also be sent to
the IIO mailing list.
Signed-off-by: David Lechner <dlechner@baylibre.com>
Link: https://lore.kernel.org/r/20240322-mainline-ad7944-doc-v2-1-0923d35d5596@baylibre.comSigned-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>

MAINTAINERS

@@ -10554,6 +10554,7 @@ T:	git git://git.kernel.org/pub/scm/linux/kernel/git/jic23/iio.git
 F:	Documentation/ABI/testing/configfs-iio*
 F:	Documentation/ABI/testing/sysfs-bus-iio*
 F:	Documentation/devicetree/bindings/iio/
+F:	Documentation/iio/
 F:	drivers/iio/
 F:	drivers/staging/iio/
 F:	include/dt-bindings/iio/