[Doc] Add mention to shared section in config_for docs [ci skip]

guides/source/configuring.md

@@ -1592,13 +1592,14 @@ These configuration points are then available through the configuration object:
 
 You can also use `Rails::Application.config_for` to load whole configuration files:
 
-  ```ruby
+  ```yaml
   # config/payment.yml:
   production:
     environment: production
     merchant_id: production_merchant_id
     public_key:  production_public_key
     private_key: production_private_key
+
   development:
     environment: sandbox
     merchant_id: development_merchant_id
@@ -1616,6 +1617,28 @@ You can also use `Rails::Application.config_for` to load whole configuration fil
   ```ruby
   Rails.configuration.payment['merchant_id'] # => production_merchant_id or development_merchant_id
   ```
+`Rails::Application.config_for` supports a `shared` configuration to group common
+configurations. The shared configuration will be merged into the environment
+configuration.
+
+  ```yaml
+  # config/example.yml
+  shared:
+    foo:
+      bar:
+        baz: 1
+
+  development:
+    foo:
+      bar:
+        qux: 2
+  ```
+
+
+  ```ruby
+  # development environment
+  Rails.application.config_for(:example)[:foo][:bar] #=> { baz: 1, qux: 2 }
+  ```
 
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railties/lib/rails/application.rb

@@ -206,12 +206,13 @@ def message_verifier(verifier_name)
 
     # Convenience for loading config/foo.yml for the current Rails env.
     #
-    # Example:
+    # Examples:
     #
     #     # config/exception_notification.yml:
     #     production:
     #       url: http://127.0.0.1:8080
     #       namespace: my_app_production
+    #
     #     development:
     #       url: http://localhost:3001
     #       namespace: my_app_development
@@ -220,6 +221,24 @@ def message_verifier(verifier_name)
     #     Rails.application.configure do
     #       config.middleware.use ExceptionNotifier, config_for(:exception_notification)
     #     end
+    #
+    #     # You can also store configurations in a shared section which will be
+    #     # merged with the environment configuration
+    #
+    #     # config/example.yml
+    #     shared:
+    #       foo:
+    #         bar:
+    #           baz: 1
+    #
+    #     development:
+    #       foo:
+    #         bar:
+    #           qux: 2
+    #
+    #     # development environment
+    #     Rails.application.config_for(:example)[:foo][:bar]
+    #     # => { baz: 1, qux: 2 }
     def config_for(name, env: Rails.env)
       yaml = name.is_a?(Pathname) ? name : Pathname.new("#{paths["config"].existent.first}/#{name}.yml")