System.IO.Ports.SerialPort.PortName should start with "/dev/" on Linux

In , a user constructed ` System.IO.Ports.SerialPort` with "ttyACM0" as `PortName` on Linux and then wondered why it did not work. It would be useful to document that the port name on Linux should be an absolute path such as "/dev/ttyACM0", rather than only the basename "ttyACM0". This would go on the following pages:

- [SerialPort Constructors](https://docs.microsoft.com/dotnet/api/system.io.ports.serialport.-ctor?view=dotnet-plat-ext-3.1), in the description of the `portName` parameter of each constructor that has one
- [SerialPort.PortName Property](https://docs.microsoft.com/en-us/dotnet/api/system.io.ports.serialport.portname?view=dotnet-plat-ext-3.1)

The implementation of the SerialPort.GetPortNames() method appears to return names in the correct format already.