added comments to note customized error msg

Author: AngelAngelXie

packages/auth/src/core/errors.ts

@@ -384,6 +384,15 @@ function _debugErrorMap(): ErrorMap<AuthErrorCode> {
       'The reCAPTCHA version sent to the backend is invalid.',
     [AuthErrorCode.WEB_OTP_NOT_RETRIEVED]: 
       'Web OTP code is not retrieved successfully',
+      /** 
+       * This is the default error message.
+       * This message is customized to one of the following depending on the type of error:
+       *  `Web OTP code is not fetched before timeout`
+       *  `The auto-retrieved credential or code is not defined`
+       *  `Web OTP get method failed to retrieve the code`
+       *  `Web OTP code received is incorrect`
+       *  `Web OTP is not supported`
+      */
   };
 }