[DOCS] Updates Changelog 9.x

Author: picandocodigo

CHANGELOG-9.x.md

@@ -2,7 +2,7 @@
 
 ## Gem
 
-The size of both `elasticsearch` and `elasticsearch-api` gems will be smaller, since some unnecessary files that were being included in the gem have been removed.
+The size of both `elasticsearch` and `elasticsearch-api` gems will be smaller, since some unnecessary files that were being included in the gem have been removed. There's also been a lot of old code cleanup for `9.x`.
 
 The required Ruby version is set to `2.6` to keep compatiblity wit JRuby 9.3. However, we only test the code against currently supported Ruby versions.
 
@@ -12,6 +12,11 @@ The CI build now runs tests to ensure compatibility with Elasticsearch Serverles
 
 ## Elasticsearch API
 
+* The source code is now based on `elasticsearch-specification`, so the API documentation is much more detailed and extensive.
+* Scroll APIs: Since sending the `scroll_id` as a parameter was deprecated, now it needs to be sent in the body for `clear_scroll`, `scroll`.
+* `indices.get_field_mapping` - `:fields` is a required parameter.
+* The functions in `utils.rb` that had names starting with double underscore have been renamed to remove these.
+
 ### Development
 
 #### Testing