Improve doxygen comments in code.

stm-firmware/main-cycle-counter.c

@@ -18,9 +18,19 @@
  * If not, see <http://www.gnu.org/licenses/>.
  */
 
+/**
+ * @addtogroup main-cycle-counter
+ * @{
+ */
+
 #include <reflow-controller/main-cycle-counter.h>
 #include <helper-macros/helper-macros.h>
 
+/**
+ * @brief Variable storing the main cycle counter.
+ * @note This variable should not be accessed directly.
+ * Use the main_cycle_counter_get() or main_cycle_counter_inc() functions.
+ */
 static uint64_t IN_SECTION(.ccm.bss) main_cycle_counter;
 
 void main_cycle_counter_init(void)
@@ -37,3 +47,5 @@ uint64_t main_cycle_counter_get(void)
 {
 	return main_cycle_counter;
 }
+
+/** @} */