[doc][doxygen] Fix doxygen struct error.

Signed-off-by: supperthomas <78900636@qq.com>

documentation/doxygen/kernel.h

@@ -32,10 +32,10 @@
  * round-robin scheduling is used for this case.
  * - The time of scheduler to choose the next highest ready thread is determinant.
  * - There are four status in thread management
- *	-# Initialization
- *	-# Running/Ready
- *	-# Blocked
- *	-# Closed
+ *  -# Initialization
+ *  -# Running/Ready
+ *  -# Blocked
+ *  -# Closed
  * - The number of threads in the system is unlimited, only related with RAM.
  */
 
@@ -95,6 +95,15 @@
  * - The send action for communication facilities is also safe for interrupt service routine.
  */
 
+/**
+ * @defgroup Signal Signal
+ * @brief signal is used for thread kill etc.
+ *
+ * A signal (also known as a soft interrupt signal), from a software perspective,
+ * is a simulation of interrupt mechanism. When it comes to its principle,
+ * thread receiving a signal is similar to processor receiving an interrupt request.
+ */
+
 /**
  * @defgroup MM Memory Management
  * @brief memory management for memory pool and heap memory