docs: be more precise in flex-model field descriptions for StorageScheduler

[nhoening]

Description

More clarity on some flex-model field descriptions, the idea came up when someone on the mailing list used soc-min and then soc-at-start below that, which is as of now infeasible for our storage scheduler.

I'm not quite happy with this PR yet, either.

Apparently, we are very hard on the soc-min and soc-max values, while we are (planning to be) softer on soc-minima, soc-maxima and soc-targets. I notice that the terms "min"/"minima" and "max"/"maxima" don't give that impression.

Example from a user perspective: If I prefer my EV battery to not be charged below 20%, I set that as soc-min. My intention is not that I bring the EV back to the charger at 10% and no schedule will be made until I bring it back up to 20%.