MDL-58551 config: add alternative_file_system_class description

config-dist.php

@@ -853,7 +853,19 @@
 // Unoconv is used convert between file formats supported by LibreOffice.
 // Use a recent version of unoconv ( >= 0.7 ), older versions have trouble running from a webserver.
 //      $CFG->pathtounoconv = '';
-
+//
+//=========================================================================
+// 14. ALTERNATIVE FILE SYSTEM SETTINGS
+//=========================================================================
+//
+// Alternative file system.
+// Since 3.3 it is possible to override file_storage and file_system API and use alternative storage systems (e.g. S3,
+// Rackspace Cloud Files, Google Cloud Storage, Azure Storage, etc.).
+// To set the alternative file storage system in config.php you can use the following setting, providing the
+// alternative system class name that will be auto-loaded by file_storage API.
+//
+//      $CFG->alternative_file_system_class = '\\local_myfilestorage\\file_system';
+//
 //=========================================================================
 // ALL DONE!  To continue installation, visit your main page with a browser
 //=========================================================================