message: add basic rule for custom function
diff: @@ -764,6 +764,28 @@ Survey.FunctionFactory.Instance.register("age", age);
  You may write, register, and use your own functions.
  +1. function name have to be camelCase.
  +2. arguments object are passed to function as single parameter during run.
  +
  +in expression, if `debugFunc({Q1}, {Q2})` are called, the `debugFunc` should have a shape of
  +
  +```
  +function debugFunc(params) {
  + let q1_value = params[0];
  + let q1_value = params[1];
  + ...
  +}
  +```
  +instead of:
  +```
  +function debugFunc(q1_value, q2_value) {
  +}
  +```
  +
  +The second form does not work.
  +
  +
  +
  Starting with v1.0.21, you can access to your survey object as `this.survey` inside a custom function.
  As result you may, for example, pass a question name to your function: `myFunc('myQuestionName')` and then get it as:
  