populate remaining missing return types, using native returns where possible

this is done separately to stable so that stable changes can be integrated easily using an empty merge.

src/scheduler/Task.php

@@ -72,6 +72,8 @@ abstract class Task{
 
 	/**
 	 * Actions to execute if the Task is cancelled
+	 *
+	 * @return void
 	 */
 	public function onCancel(){