Add changelog for #32956 [ci skip]

Add mention about default value of `config.active_model.i18n_full_message`.

activemodel/CHANGELOG.md

+*   Add `config.active_model.i18n_full_message` in order to control whether
+    the `full_message` error format can be overridden at the attribute or model
+    level in the locale files. This is `false` by default.
+
+    *Martin Larochelle*
+
 *   Rails 6 requires Ruby 2.4.1 or newer.
 
     *Jeremy Daer*

guides/source/configuring.md

@@ -307,7 +307,7 @@ All these configuration options are delegated to the `I18n` library.
 
 ### Configuring Active Model
 
-* `config.active_model.i18n_full_message` is a boolean value which controls whether the `full_message` error format can be overridden at the attribute or model level in the locale files.
+* `config.active_model.i18n_full_message` is a boolean value which controls whether the `full_message` error format can be overridden at the attribute or model level in the locale files. This is `false` by default.
 
 ### Configuring Active Record