Issue #2831630 by quietone, Jo Fitzgerald, Kifah Meeran, phenaproxima,...

Issue #2831630 by quietone, Jo Fitzgerald, Kifah Meeran, phenaproxima, gaurav.kapoor, mikeryan, heddn, xjm, claudiu.cristea: Add documentation to download process plugin

core/modules/migrate/src/Plugin/migrate/process/Download.php

@@ -12,7 +12,42 @@
 use Symfony\Component\DependencyInjection\ContainerInterface;
 
 /**
- * Downloads a file from a remote location into the local file system.
+ * Downloads a file from a HTTP(S) remote location into the local file system.
+ *
+ * The source value is an array of two values:
+ * - source URL, e.g. 'http://www.example.com/img/foo.img'
+ * - destination URI, e.g. 'public://images/foo.img'
+ *
+ * Available configuration keys:
+ * - rename: (optional) If set, a unique destination URI is generated. If not
+ *   set, the destination URI will be overwritten if it exists.
+ * - guzzle_options: (optional)
+ *   @link http://docs.guzzlephp.org/en/latest/request-options.html Array of request options for Guzzle. @endlink
+ *
+ * Examples:
+ *
+ * @code
+ * process:
+ *   plugin: download
+ *   source:
+ *     - source_url
+ *     - destination_uri
+ * @endcode
+ *
+ * This will download source_url to destination_uri.
+ *
+ * @code
+ * process:
+ *   plugin: download
+ *   source:
+ *     - source_url
+ *     - destination_uri
+ *   rename: true
+ * @endcode
+ *
+ * This will download source_url to destination_uri and ensure that the
+ * destination URI is unique. If a file with the same name exists at the
+ * destination, a numbered suffix like '_0' will be appended to make it unique.
  *
  * @MigrateProcessPlugin(
  *   id = "download"