diff --git a/modules/system/system.api.php b/modules/system/system.api.php
index ec7f6b8..578a1b8 100644
--- a/modules/system/system.api.php
+++ b/modules/system/system.api.php
@@ -1994,21 +1994,27 @@ function hook_permission() {
  * @return array
  *   An associative array of theme hook information. The keys on the outer
  *   array are the internal names of the hooks, and the values are arrays
- *   containing information about the hook. Each array may contain the
- *   following elements:
- *   - variables: (required if "render element" not present) An array of
- *     variables that this theme hook uses. This value allows the theme layer
- *     to properly utilize templates. Each array key represents the name of the
- *     variable and the value will be used as the default value if it is not
- *     given when theme() is called. Template implementations receive these
- *     arguments as variables in the template file. Function implementations
- *     are passed this array data in the $variables parameter.
- *   - render element: (required if "variables" not present) A string that is
- *     the name of the sole renderable element to pass to the theme function.
- *     The string represents the name of the "variable" that will hold the
- *     renderable array inside any optional preprocess or process functions.
- *     Cannot be used with the "variables" item; only one or the other, not
- *     both, can be present in a hook's info array.
+ *   containing information about the hook. Each information array must contain
+ *   either a 'variables' element or a 'render element' element, but not both.
+ *   Use 'variables' if you are theming an item that has inputs for the data and
+ *   style; in this case, the variables not supplied by the calling function
+ *   will be given default values and passed to the template or theme function.
+ *   Use 'render element' if you are theming a single element or element tree
+ *   composed of elements defined in hook_element() implementations, such as a
+ *   form array, a page array, or a single checkbox element. The theme
+ *   information arrays can contain the following elements:
+ *   - variables: (see above) An array of variables that this theme hook uses
+ *     to determine how to render its inputs, such as the data to be rendered
+ *     and styling variables. Each array key is the name of the variable, and
+ *     the value given is used as the default value if the function calling
+ *     theme() does not supply it. Template implementations receive each array
+ *     key as a variable in the template file (so they must be legal PHP
+ *     variable names). Function implementations are passed the variables in a
+ *     single $variables function argument.
+ *   - render element: (see above) The name of the renderable element or element
+ *     tree to pass to the theme function. This name is used as the name of the
+ *     variable that holds the renderable element or tree in preprocess and
+ *     process functions.
  *   - file: The file the implementation resides in. This file will be included
  *     prior to the theme being rendered, to make sure that the function or
  *     preprocess function (as needed) is actually loaded; this makes it