Document the Filter annotation class

Follow-up for #1868772-105: Convert filters to plugins

+++ b/core/modules/filter/lib/Drupal/filter/Annotation/Filter.phpundefined
@@ -0,0 +1,26 @@
+class Filter extends Plugin {
+
+  public $title;
+  public $description = '';
+  public $weight = 0;
+  public $status = FALSE;
+  public $cache = TRUE;
+  public $settings = array();
+
+}