DOCSP-43242: Improve UTF-8 validation documentation to clarify validation occurs on decoded data only

source/fundamentals/bson/utf8-validation.txt

@@ -25,15 +25,16 @@ processing overhead since it needs to check the data.
 If you *disable* validation, your application avoids the validation processing
 overhead, but cannot guarantee consistent presentation of invalid UTF-8 data.
 
-The driver enables UTF-8 validation by default. It checks documents for any
-characters that are not encoded in a valid UTF-8 format when it transfers data
-between your application and MongoDB.
+By default, the driver enables UTF-8 validation on data from MongoDB. 
+It checks incoming documents for any characters that are not encoded in a 
+valid UTF-8 format when it parses data sent from MongoDB to your application.
 
 .. note::
 
-   The current version of the {+driver-short+} automatically substitutes
-   invalid UTF-8 characters with alternate valid UTF-8 ones before
-   validation when you send data to MongoDB. Therefore, the validation
+   The current version of the {+driver-short+} automatically substitutes invalid
+   `lone surrogates <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String#utf-16_characters_unicode_code_points_and_grapheme_clusters>`__
+   with the `replacement character <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/toWellFormed>`__ 
+   before validation when you send data to MongoDB. Therefore, the validation
    only throws an error when the setting is enabled and the driver
    receives invalid UTF-8 document data from MongoDB.