How to save a user identifier with survey results - Add information on how to do so

[JaneSjs]

T17441 - identifying the surveyed person
https://surveyjs.answerdesk.io/internal/ticket/details/T17441

---

• To save an additional data to survey results, it is necessary to use the survey.setValue('userId', value) function. Please add this information to our docs.
• It is a bit unclear what the variables are used for (variables). I see that they are not saved to survey results (View Demo). Please clarify this.