doc: introduce doxygen group "threshold"

This patch introduces a doxygen group to put together the documentation
relative to threshold. Group appear in a separate page and they can have
their own documentation. This is useful when a feature is splitted into
different files.

src/detect-engine-threshold.c

@@ -15,6 +15,17 @@
  * 02110-1301, USA.
  */
 
+/**
+ * \defgroup threshold Thresholding
+ *
+ * This feature is used to reduce the number of logged alerts for noisy rules.
+ * This can be tuned to significantly reduce false alarms, and it can also be
+ * used to write a newer breed of rules. Thresholding commands limit the number
+ * of times a particular event is logged during a specified time interval.
+ *
+ * @{
+ */
+
 /**
  * \file
  *
@@ -658,3 +669,6 @@ void ThresholdContextDestroy(DetectEngineCtx *de_ctx)
         SCFree(de_ctx->ths_ctx.th_entry);
 }
 
+/**
+ * @}
+ */

src/detect-threshold.c

@@ -15,12 +15,20 @@
  * 02110-1301, USA.
  */
 
+/**
+ * \ingroup threshold
+ * @{
+ */
+
 /**
  * \file
  *
  * \author Breno Silva <breno.silva@gmail.com>
  *
- * Implements the threshold keyword
+ * Implements the threshold keyword.
+ *
+ * The feature depends on what is provided
+ * by detect-engine-threshold.c and util-threshold-config.c
  */
 
 #include "suricata-common.h"
@@ -878,3 +886,7 @@ void ThresholdRegisterTests(void) {
     UtRegisterTest("DetectThresholdTestSig6Ticks", DetectThresholdTestSig6Ticks, 1);
 #endif /* UNITTESTS */
 }
+
+/**
+ * @}
+ */

src/util-threshold-config.c

@@ -15,6 +15,11 @@
  * 02110-1301, USA.
  */
 
+/**
+ * \ingroup threshold
+ * @{
+ */
+
 /**
  * \file
  *
@@ -1707,3 +1712,6 @@ void SCThresholdConfRegisterTests(void)
 #endif /* UNITTESTS */
 }
 
+/**
+ * @}
+ */