diff --git a/includes/common.inc b/includes/common.inc
index 1ef681f..48fb746 100644
--- a/includes/common.inc
+++ b/includes/common.inc
@@ -5508,6 +5508,11 @@ function drupal_render_page($page) {
  *
  * Recursively iterates over each of the array elements, generating HTML code.
  *
+ * Renderable arrays have two kinds of key/value pairs: properties and
+ * children. Properties have keys starting with '#' and their values influence
+ * how the array will be rendered. Children are all elements whose keys do not
+ * start with a '#'. Their values should be renderable arrays themselves.
+ *
  * HTML generation is controlled by two properties containing theme functions,
  * #theme and #theme_wrappers.
  *