Issue #1716900 by chx: Clarify why hook_exit() must not print.

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