bridge: Elaborate a bit on dumb VGA bridges in Kconfig

It's better if we explain a bit that this pertains to
non-programmable VGA DAC bridges.
Reviewed-by: NLaurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20180302091426.8463-1-linus.walleij@linaro.org

drivers/gpu/drm/bridge/Kconfig

@@ -30,7 +30,8 @@ config DRM_DUMB_VGA_DAC
 	depends on OF
 	select DRM_KMS_HELPER
 	help
-	  Support for RGB to VGA DAC based bridges
+	  Support for non-programmable RGB to VGA DAC bridges, such as ADI
+	  ADV7123, TI THS8134 and THS8135 or passive resistor ladder DACs.
 
 config DRM_LVDS_ENCODER
 	tristate "Transparent parallel to LVDS encoder support"