docs: Add private class features info to no-underscore-dangle (#17386)

docs/src/rules/no-underscore-dangle.md

@@ -3,16 +3,17 @@ title: no-underscore-dangle
 rule_type: suggestion
 ---
 
-
 As far as naming conventions for identifiers go, dangling underscores may be the most polarizing in JavaScript. Dangling underscores are underscores at either the beginning or end of an identifier, such as:
 
 ```js
 var _foo;
 ```
 
-There is actually a long history of using dangling underscores to indicate "private" members of objects in JavaScript (though JavaScript doesn't have truly private members, this convention served as a warning). This began with SpiderMonkey adding nonstandard methods such as `__defineGetter__()`. The intent with the underscores was to make it obvious that this method was special in some way. Since that time, using a single underscore prefix has become popular as a way to indicate "private" members of objects.
+There is a long history of marking "private" members with dangling underscores in JavaScript, beginning with SpiderMonkey adding nonstandard methods such as `__defineGetter__()`. Since that time, using a single underscore prefix has become the most popular convention for indicating a member is not part of the public interface of an object.
+
+It is recommended to use the formal [private class features](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Classes/Private_class_fields) introduced in ECMAScript 2022 for encapsulating private data and methods rather than relying on naming conventions.
 
-Whether or not you choose to allow dangling underscores in identifiers is purely a convention and has no effect on performance, readability, or complexity. It's purely a preference.
+Allowing dangling underscores in identifiers is purely a convention and has no effect on performance, readability, or complexity. They do not have the same encapsulation benefits as private class features, even with this rule enabled.
 
 ## Rule Details