[kernel] Improve code comments.

Author: Guozhanxin

src/clock.c

@@ -130,7 +130,7 @@ RTM_EXPORT(rt_tick_from_millisecond);
 /**
  * @brief    This function will return the passed millisecond from boot.
  *
- * @note     When the value of RT_TICK_PER_SECOND is lower than 1000 or
+ * @note     if the value of RT_TICK_PER_SECOND is lower than 1000 or
  *           is not an integral multiple of 1000, this function will not
  *           provide the correct 1ms-based tick.
  *

src/components.c

@@ -78,7 +78,9 @@ static int rti_end(void)
 INIT_EXPORT(rti_end, "6.end");
 
 /**
- * @brief  RT-Thread Components Initialization for board.
+ * @brief  Onboard components initialization. In this function, the board-level
+ *         initialization function will be called to complete the initialization
+ *         of the on-board peripherals.
  */
 void rt_components_board_init(void)
 {
@@ -225,9 +227,8 @@ void rt_application_init(void)
 }
 
 /**
- * @brief  RT-Thread startup function. This function will call all levels of initialization
- *         functions to complete the initialization of the system, and finally start the
- *         scheduler.
+ * @brief  This function will call all levels of initialization functions to complete
+ *         the initialization of the system, and finally start the scheduler.
  */
 int rtthread_startup(void)
 {

src/cpu.c

@@ -69,7 +69,6 @@ static void _cpu_preempt_enable(void)
  * @brief   Initialize a static spinlock object.
  *
  * @param   lock is a pointer to the spinlock to initialize.
- *            It is assumed that storage for the spinlock will be allocated in your application.
  */
 void rt_spin_lock_init(struct rt_spinlock *lock)
 {