Improve `VoltageTransformer` documentation

[llucax]

What's needed?

We need to make the documentation of VoltageTransformer primary and secondary fields, at the docs mention they are voltages, making it a bit confusing.

Proposed solution

Improve the docs to mention the ratios and make it clear that primary and secondary are not actually voltages but number of coil turns or something like that.

Additional context

Coming from:

• Documentation improvements ideas #239