Improve `BatteryPool` docs about streaming soc and capacity independent of inverter status

[shsms]

[shsms]

I just remembered when writing the issue, that this is behaving as designed. So I decided to write this note instead:

BatteryPool is expected to report only the usable SoC and Capacity of the pool. And if a corresponding inverter is out of order, then that capacity and SoC are not usable, so the values from such batteries should not be considered.

I will keep this issue open until the next SDK meeting, just in case.

[thomas-nicolai-frequenz]

I think this should be clearly document in case this isn't yet.

And if a corresponding inverter is out of order, then that capacity and SoC are not usable

That totally makes sense imho

[llucax]

This wasn't mentioned in the last meeting, maybe you want to add it to the agenda @shsms ?