power: supply: Explain maintenance charging

In order for everyone to understand clearly why we want to use
maintenance charging for batteries, expand the description with two
diagrams and some text.

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