Documentation of System.Diagnostics.Switch should properly describe how to implement a derived class.

All existing derived classes of `System.Diagnostics.Switch` save their state in `SwitchSetting` and override `OnValueChanged` to have `SwtichSetting` properly reflect the corresponding setting.

The documentation of `System.Diagnostics.Switch` however describes the process writing an inherited class as:
> When inheriting from this class, you must implement the `SwitchSetting` method

The problem here is that `SwitchSetting` isn't even `virtual`.