add documentation for config.active_support.use_authenticated_message_encryption [ci skip]

related to https://github.com/rails/rails/issues/33170

guides/source/configuring.md

@@ -707,6 +707,8 @@ There are a few configuration options available in Active Support:
 
 * `config.active_support.use_sha1_digests` specifies whether to use SHA-1 instead of MD5 to generate non-sensitive digests, such as the ETag header. Defaults to false.
 
+* `config.active_support.use_authenticated_message_encryption` specifies whether to use AES-256-GCM authenticated encryption as the default cipher for encrypting messages instead of AES-256-CBC. This is false by default, but enabled when loading defaults for Rails 5.2.
+
 * `ActiveSupport::Logger.silencer` is set to `false` to disable the ability to silence logging in a block. The default is `true`.
 
 * `ActiveSupport::Cache::Store.logger` specifies the logger to use within cache store operations.