Documentation: ABI: leds-turris-omnia: document sysfs attribute

Document the global brightness attribute for the Turris Omnia LED
controller.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>

diff --git a/Documentation/ABI/testing/sysfs-class-led-driver-turris-omnia b/Documentation/ABI/testing/sysfs-class-led-driver-turris-omnia
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+What:          /sys/class/leds/<led>/device/brightness
+Date:          July 2020
+KernelVersion: 5.9
+Contact:       Marek Behún <marek.behun@nic.cz>
+Description:   (RW) On the front panel of the Turris Omnia router there is also
+               a button which can be used to control the intensity of all the
+               LEDs at once, so that if they are too bright, user can dim them.
+
+               The microcontroller cycles between 8 levels of this global
+               brightness (from 100% to 0%), but this setting can have any
+               integer value between 0 and 100. It is therefore convenient to be
+               able to change this setting from software.
+
+               Format: %i