diff --git a/core/modules/system/system.api.php b/core/modules/system/system.api.php
index 3d2cae0..4eac342 100644
--- a/core/modules/system/system.api.php
+++ b/core/modules/system/system.api.php
@@ -273,7 +273,8 @@ 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