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>