Add documentation for Date variables after the new API has been added to documentation.

```
const variables = new VariableService();

variables.set('manipulatedDate', new Date("2018-01-23T14:42:45.435+0200"));

variables.setTyped("manipulatedDate", {
value: "2018-01-23T14:42:45.435+0200",
type: "Date",
valueInfo:

{ objectTypeName: "Date", serializationDataFormat: "application/json" }

});
```