doc: kernel: Add sys_sem APIs to kernel documentation

The kernel documentation was missing sys_sem APIs.

Signed-off-by: Jukka Rissanen <jukka.rissanen@linux.intel.com>

doc/reference/kernel/synchronization/semaphores.rst

@@ -130,3 +130,13 @@ API Reference
 
 .. doxygengroup:: semaphore_apis
    :project: Zephyr
+
+User Mode Semaphore API Reference
+*********************************
+
+The sys_sem exists in user memory working as counter semaphore for user mode
+thread when user mode enabled. When user mode isn't enabled, sys_sem behaves
+like k_sem.
+
+.. doxygengroup:: user_semaphore_apis
+   :project: Zephyr

include/sys/sem.h

@@ -39,6 +39,12 @@ struct sys_sem {
 #endif
 };
 
+/**
+ * @defgroup user_semaphore_apis User mode semaphore APIs
+ * @ingroup kernel_apis
+ * @{
+ */
+
 /**
  * @brief Statically define and initialize a sys_sem
  *
@@ -131,6 +137,10 @@ int sys_sem_take(struct sys_sem *sem, k_timeout_t timeout);
  */
 unsigned int sys_sem_count_get(struct sys_sem *sem);
 
+/**
+ * @}
+ */
+
 #ifdef __cplusplus
 }
 #endif