Document home battery smart grid charging not controlled by load management (but handled during dimming)

[andig]

my home battery can draw up to 9 kW

evcc is not aware of that. Blatantly: grid charging does not participate in load management. If dimmed (§14a) we'll prevent grid-charging, but not under general load management.

/cc @premultiply

Originally posted by @andig in #24567