Documentation: leds: Add description of LED Flash class extension

The documentation being added contains overall description of the
LED Flash Class and the related sysfs attributes.

Signed-off-by: Jacek Anaszewski <j.anaszewski@samsung.com>
Acked-by: Kyungmin Park <kyungmin.park@samsung.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

diff --git a/Documentation/leds/leds-class-flash.txt b/Documentation/leds/leds-class-flash.txt
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+++ b/Documentation/leds/leds-class-flash.txt
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+
+Flash LED handling under Linux
+==============================
+
+Some LED devices provide two modes - torch and flash. In the LED subsystem
+those modes are supported by LED class (see Documentation/leds/leds-class.txt)
+and LED Flash class respectively. The torch mode related features are enabled
+by default and the flash ones only if a driver declares it by setting
+LED_DEV_CAP_FLASH flag.
+
+In order to enable the support for flash LEDs CONFIG_LEDS_CLASS_FLASH symbol
+must be defined in the kernel config. A LED Flash class driver must be
+registered in the LED subsystem with led_classdev_flash_register function.
+
+Following sysfs attributes are exposed for controlling flash LED devices:
+(see Documentation/ABI/testing/sysfs-class-led-flash)
+       - flash_brightness
+       - max_flash_brightness
+       - flash_timeout
+       - max_flash_timeout
+       - flash_strobe
+       - flash_fault