extcon: Add documentation for EXTCON_CHG_USB_* and EXTCON_USB_*

Current there is both "EXTCON_USB" and "EXTCON_CHG_USB_SDP" which
both seem to suggest a standard downstream port. But there is no
documentation describing how these relate.

Thus add documentation to describe EXTCON_CHG_USB_SDP should always
appear together with EXTCON_USB, and EXTCON_CHG_USB_ACA would normally
appear with EXTCON_USB_HOST.
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>

include/linux/extcon.h

@@ -46,7 +46,14 @@
 #define EXTCON_USB		1
 #define EXTCON_USB_HOST		2
 
-/* Charging external connector */
+/*
+ * Charging external connector
+ *
+ * When one SDP charger connector was reported, we should also report
+ * the USB connector, which means EXTCON_CHG_USB_SDP should always
+ * appear together with EXTCON_USB. The same as ACA charger connector,
+ * EXTCON_CHG_USB_ACA would normally appear with EXTCON_USB_HOST.
+ */
 #define EXTCON_CHG_USB_SDP	5	/* Standard Downstream Port */
 #define EXTCON_CHG_USB_DCP	6	/* Dedicated Charging Port */
 #define EXTCON_CHG_USB_CDP	7	/* Charging Downstream Port */