diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index 79ac8f169d6e2d67842537a5215fc59fbe766ae8..fbb0a554fb0b82ef79ffd9f60aa40e9589a4d3bb 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -689,7 +689,8 @@ function hook_element_info_alter(&$type) {
  * Perform cleanup tasks.
  *
  * This hook is run at the end of each page request. It is often used for
- * page logging and specialized cleanup. This hook MUST NOT print anything.
+ * page logging and specialized cleanup. This hook MUST NOT print anything
+ * because by the time it runs the response is already sent to the browser.
  *
  * Only use this hook if your code must run even for cached page views.
  * If you have code which must run once on all non-cached pages, use