cron: update docs to clarify purpose

modules/cron/docs/cron.md

 Cron
 =============================
 
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 [TOC]
 
 Introduction
@@ -17,6 +10,13 @@ The cron module allows you to do tasks regularly, by setting up a cron
 job that calls a hook in SimpleSAMLphp.  This will invoke
 `hooks/hook_cron.php` in any enabled modules
 
+Modules, like e.g. the metarefresh module, that want to invoke tasks
+on set times, can register cron hooks for use by this module. You in turn
+configure your system's `cron` or other time based execution functionality
+to periodically trigger this module's endpoint, so all registered hooks
+will be executed at that time. This module also takes care of reporting
+back the result and provides an interface in SimpleSAMLphp's admin
+panel to trigger jobs manually.
 
 Preparations
 ------------