update changelog with new stabilisation

Author: scampi

CHANGELOG.md

@@ -2,9 +2,12 @@
 
 ## [Unreleased]
 
+### Changed
+
 - Change option `format_doc_comment` to `format_code_in_doc_comment`.
 - `use_small_heuristics` changed to be an enum and stabilised. Configuration
   options are now ready for 1.0.
+- Stabilise `fn_args_density` configuration option #3581
 
 ## [1.2.2] 2019-04-24
 

Processes.md

@@ -17,6 +17,7 @@ Open a pull request that closes the tracking issue. The tracking issue is listed
 
 - Update the `Config` enum marking the option as stable.
 - Update the the `Configuration.md` file marking the option as stable.
+- Update `CHANGELOG.md` marking the option as stable.
 
 ## After the stabilisation