Index: src/wp-includes/user.php
===================================================================
--- src/wp-includes/user.php	(revision 37066)
+++ src/wp-includes/user.php	(working copy)
@@ -414,11 +414,14 @@
 //
 
 /**
- * Get the current user's ID
+ * Retrieves the current user's ID.
  *
+ * Should be used only after the {@see 'init'} action has fired, as the current user
+ * is not set before that point in the loading order.
+ *
  * @since MU
  *
- * @return int The current user's ID
+ * @return int The current user's ID.
  */
 function get_current_user_id() {
 	if ( ! function_exists( 'wp_get_current_user' ) )