power: supply: core: Add kerneldoc to battery struct

This complements the struct power_supply_battery_info with
extensive kerneldoc explaining the different semantics of the
fields, including an overview of the CC/CV charging concepts
implicit in some of the struct members.

This is done to first establish semantics before I can
add more charging methods by breaking out the CC/CV parameters
to its own struct.

Tested-by: Randy Dunlap <rdunlap@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Matti Vaittinen <matti.vaittinen@fi.rohmeurope.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>