[feature/system-cron] Added @param/@return documentation

Also adjusted some function descriptions for greater informativity.

PHPBB3-9596

phpBB/includes/cron/task/task.php

@@ -49,6 +49,15 @@ interface phpbb_cron_task
 	/**
 	* Returns whether this cron task can be run in shutdown function.
 	*
+	* By the time shutdown sequence invokes a particular piece of code,
+	* resources that that code requires may already be released.
+	* If so, a particular cron task may be marked shutdown function-
+	* unsafe, and it will be executed in normal program flow.
+	*
+	* Generally speaking cron tasks should start off as shutdown function-
+	* safe, and only be marked shutdown function-unsafe if a problem
+	* is discovered.
+	*
 	* @return bool
 	*/
 	public function is_shutdown_function_safe();