power: supply: bq25890: Document POWER_SUPPLY_PROP_CURRENT_NOW

Document that POWER_SUPPLY_PROP_CURRENT_NOW really does refer to ADC-sampled
immediate battery charge current I_BAT , since the meaning is not clear with
all the currents which might be measured by charger chips.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>