backlight: backlight: Add overview and update existing doc

Add overview chapter to backlight.c.

Update existing kernel-doc to follow a more consistent style and drop
kernel-doc for deprecated functions.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>