Issue #1250500 by joachim, jhodgdon: Add defgroup for callback functions

core/includes/module.inc

@@ -617,11 +617,42 @@ function module_uninstall($module_list = array(), $uninstall_dependents = TRUE)
  * just models that you can modify. Only the hooks implemented within modules
  * are executed when running Drupal.
  *
- * See also @link themeable the themeable group page. @endlink
+ * @see themeable
+ * @see callbacks
  *
  * @} End of "defgroup hooks".
  */
 
+ /**
+  * @defgroup callbacks Callbacks
+  * @{
+  * Callback function signatures.
+  *
+  * Drupal's API sometimes uses callback functions to allow you to define how
+  * some type of processing happens. A callback is a function with a defined
+  * signature, which you define in a module. Then you pass the function name as
+  * a parameter to a Drupal API function or return it as part of a hook
+  * implementation return value, and your function is called at an appropriate
+  * time. For instance, when setting up batch processing you might need to
+  * provide a callback function for each processing step and/or a callback for
+  * when processing is finished; you would do that by defining these functions
+  * and passing their names into the batch setup function.
+  *
+  * Callback function signatures, like hook definitions, are described by
+  * creating and documenting dummy functions in a *.api.php file; normally, the
+  * dummy callback function's name should start with "callback_", and you should
+  * document the parameters and return value and provide a sample function body.
+  * Then your API documentation can refer to this callback function in its
+  * documentation. A user of your API can usually name their callback function
+  * anything they want, although a standard name would be to replace "callback_"
+  * with the module name.
+  *
+  * @see hooks
+  * @see themeable
+  *
+  * @} End of "defgroup callbacks".
+  */
+
 /**
  * Returns an array of modules required by core.
  */

core/modules/system/theme.api.php

@@ -62,6 +62,8 @@
  *
  * @see theme()
  * @see hook_theme()
+ * @see hooks
+ * @see callbacks
  *
  * @} End of "defgroup themeable".
  */