extcon: Add documentation for EXTCON_CHG_USB_SLOW/FAST

Currently there are no documentation for EXTCON_CHG_USB_SLOW/FAST
charger connector. These names don't mean much and no guide to tell
users how to use it, thus try to add documentation to make them clear.

Suggested-by: NeilBrown <neilb@suse.com>
Signed-off-by: Baolin Wang <baolin.wang@linaro.org>
[cw00.choi: Use the 'connector' expression instead of 'cable']
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>