Issue #2877483: Coding standards: Switch from @section to @defgroup

@section is for sections within a docbock. @defgroup can be used to
group related functions together.

fillpdf.rules.inc

@@ -6,7 +6,8 @@
  */
 
 /**
- * @section Rules hooks
+ * @defgroup rules_hooks Rules hooks
+ * @{
  */
 
 /**
@@ -155,7 +156,12 @@ function fillpdf_rules_condition_info() {
 }
 
 /**
- * @section Rules callbacks
+ * @} End of "defgroup rules_hooks".
+ */
+
+/**
+ * @defgroup rules_callbacks Rules callbacks
+ * @{
  */
 
 /**
@@ -219,7 +225,12 @@ function fillpdf_rules_action_delete_file($filename) {
 }
 
 /**
- * @section Condition callbacks
+ * @} End of "defgroup rules_callbacks".
+ */
+
+/**
+ * @defgroup condition_callbacks Condition callbacks
+ * @{
  */
 
 /**
@@ -228,3 +239,7 @@ function fillpdf_rules_action_delete_file($filename) {
 function _fillpdf_rules_metadata_info() {
   return array();
 }
+
+/**
+ * @} End of "defgroup condition_callbacks".
+ */