docs: add comments for ExpressionErrors.doubleProto [ci-skip]

packages/babel-parser/src/parser/util.js

@@ -286,10 +286,15 @@ export default class UtilParser extends Tokenizer {
 }
 
 /**
- * The Expression Errors is a context struct used to track
+ * The ExpressionErrors is a context struct used to track
  * - **shorthandAssign**: track initializer `=` position when parsing ambiguous
- *   patterns. When we are sure the parsed pattern is a RHS, we will throw on
- *   this position for invalid assign syntax, otherwise reset to -1
+ *   patterns. When we are sure the parsed pattern is a RHS, which means it is
+ *   not a pattern, we will throw on this position on invalid assign syntax,
+ *   otherwise it will be reset to -1
+ * - **doubleProto**: track the duplicate `__proto__` key position when parsing
+ *   ambiguous object patterns. When we are sure the parsed pattern is a RHS,
+ *   which means it is an object literal, we will throw on this position for
+ *   __proto__ redefinition, otherwise it will be reset to -1
  */
 export class ExpressionErrors {
   shorthandAssign = -1;