doc: ztest: Include ztress to API reference

Include ztress documentation to ztest API section.

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>

subsys/testsuite/ztest/include/zephyr/ztress.h

@@ -3,6 +3,13 @@
  *
  * SPDX-License-Identifier: Apache-2.0
  */
+
+/**
+ * @file
+ *
+ * @brief Zephyr testing framework ztress macros
+ */
+
 #ifndef TESTSUITE_ZTEST_INCLUDE_ZTRESS_H__
 #define TESTSUITE_ZTEST_INCLUDE_ZTRESS_H__
 
@@ -17,6 +24,15 @@ extern "C" {
 #define ZTRESS_ID_THREAD 0
 #define ZTRESS_ID_K_TIMER 1
 
+/**
+ * @defgroup ztest_ztress Ztest ztress macros
+ * @ingroup ztest
+ *
+ * This module provides test stress when using Ztest.
+ *
+ * @{
+ */
+
 /** @brief Descriptor of a k_timer handler execution context.
  *
  * The handler is executed in the k_timer handler context which typically means
@@ -237,6 +253,10 @@ int ztress_preempt_count(uint32_t id);
  */
 uint32_t ztress_optimized_ticks(uint32_t id);
 
+/**
+ * @}
+ */
+
 #ifdef __cplusplus
 }
 #endif