meters: `powers` and `voltages` not listed, direction not explained

[sicheste]

Hello,

in the documentation of the yaml, in the section of custom meters, the powers and voltages are not listed.

Also it is not described in which cases the values are expected to be positive or negative (e.g. positive power values for inverters, for grid meter negative value means excess).

Regards

[naltatis]

fixed by #1056