Documentation: leds: Add description of v4l2-flash sub-device

This patch extends LED Flash class documention by
the description of interactions with v4l2-flash sub-device.

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