power: supply: core: Add missing documentation for CHARGE_CONTROL_* properties

The existing POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT and
POWER_SUPPLY_PROP_CHARGE_CONTROL_LIMIT_MAX properties
don't have documentation. I add that documentation here.

v5 changes:
- Split this commit out from the previous two commits.

Signed-off-by: Nick Crews <ncrews@chromium.org>
Reviewed-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>