Add the Preview label

Author: mumian

articles/azure-resource-manager/bicep/file.md

@@ -97,9 +97,12 @@ The allowed values are:
 
 In a module, you can specify a scope that is different than the scope for the rest of the Bicep file. For more information, see [Configure module scope](modules.md#set-module-scope)
 
-## Functions
+## Functions (Preview)
 
-Within your Bicep file, you can create your own functions in addition to using the [standard Bicep functions](./bicep-functions.md) that are automatically available within your Bicep files. Create your own functions when you have complicated expressions that are used repeatedly in your Bicep files.
+> [!NOTE]
+> To enable the preview feature, see [Enable experimental features](./bicep-config.md#enable-experimental-features).
+
+In your Bicep file, you can create your own functions in addition to using the [standard Bicep functions](./bicep-functions.md) that are automatically available within your Bicep files. Create your own functions when you have complicated expressions that are used repeatedly in your Bicep files.
 
 ```bicep
 func buildUrl(https bool, hostname string, path string) string => '${https ? 'https' : 'http'}://${hostname}${empty(path) ? '' : '/${path}'}'