message: Add documentation for ExpressionItem class
diff: @@ -4,6 +4,9 @@ import { ISurvey } from "./base-interfaces";
  import { ILocalizableOwner, LocalizableString } from "./localizablestring";
  import { ConditionRunner } from "./conditions";
  +/**
  + * Base class for HtmlConditionItem and UrlConditionItem classes.
  + */
  export class ExpressionItem extends Base implements ILocalizableOwner {
  public locOwner: ILocalizableOwner;
  constructor(expression: string = null) {
  