Docs: Voltmeter -- Clarify port documentation

[heuer]

https://elektrofuzzis.github.io/ftSwarm/cpp/analog/analog/ states "ftSwarm is able to measure voltages at input FTSWARM_A2 as well."

https://elektrofuzzis.github.io/ftSwarm/cpp/analog/FtSwarmVoltmeter/ states "Voltage measurement can be done using input A2 on the ftSwarm."

The documentation leaves open whether the measurement is only possible on A2 or on every port including A2. The first page says "possible on A2 too" while the second page seems to say it's only possible on A2.

[heuer]

https://github.com/elektrofuzzis/ftSwarm/blob/master/arduino-cpp-library/src/ftSwarm.cpp#L250 does not restrict the usable port so I'd assume the ports A1 .. A4 work for the voltmeter?

[yet-another-fuzzi]

new documentation in place