[libc] add comments to the cstdlib.c (#8209)

Co-authored-by: sq <1838545301@qq.com>

components/libc/compilers/common/cstdlib.c

@@ -14,6 +14,11 @@
 #define DBG_LVL    DBG_INFO
 #include <rtdbg.h>
 
+/**
+ * @brief   This function is called when a thread exits. It can detach the thread and perform cleanup.
+ *
+ * @param   status is the exit status of the thread.
+ */
 void __rt_libc_exit(int status)
 {
     rt_thread_t self = rt_thread_self();
@@ -36,6 +41,13 @@ void __rt_libc_exit(int status)
 }
 
 #ifdef RT_USING_MSH
+/**
+ * @brief   Execute a command using the Micro-Shell (MSH) subsystem.
+ *
+ * @param   command is the command string to execute.
+ *
+ * @return  Returns 0 after executing the command.
+ */
 int system(const char *command)
 {
     extern int msh_exec(char *cmd, rt_size_t length);
@@ -50,6 +62,15 @@ int system(const char *command)
 RTM_EXPORT(system);
 #endif /* RT_USING_MSH */
 
+/**
+ * @brief   Convert a long integer to a string representation with a specified radix.
+ *
+ * @param   value is the long integer to convert.
+ * @param   string is the destination string where the result will be stored.
+ * @param   radix is the base of the number system to be used for conversion.
+ *
+ * @return  Returns a pointer to the destination string.
+ */
 char *ltoa(long value, char *string, int radix)
 {
     char tmp[33];
@@ -100,12 +121,29 @@ char *ltoa(long value, char *string, int radix)
     return string;
 }
 
+/**
+ * @brief   Convert an integer to a string representation with a specified radix.
+ *
+ * @param   value is the integer to convert.
+ * @param   string is the destination string where the result will be stored.
+ * @param   radix is the base of the number system to be used for conversion.
+ *
+ * @return  Returns a pointer to the destination string.
+ */
 char *itoa(int value, char *string, int radix)
 {
     return ltoa(value, string, radix);
 }
 
-
+/**
+ * @brief   Convert an unsigned long integer to a string representation with a specified radix.
+ *
+ * @param   value is the unsigned long integer to convert.
+ * @param   string is the destination string where the result will be stored.
+ * @param   radix is the base of the number system to be used for conversion.
+ *
+ * @return  Returns a pointer to the destination string.
+ */
 char *ultoa(unsigned long value, char *string, int radix)
 {
     char tmp[33];
@@ -143,6 +181,15 @@ char *ultoa(unsigned long value, char *string, int radix)
     return string;
 }
 
+/**
+ * @brief   Convert an unsigned integer to a string representation with a specified radix.
+ *
+ * @param   value is the unsigned integer to convert.
+ * @param   string is the destination string where the result will be stored.
+ * @param   radix is the base of the number system to be used for conversion.
+ *
+ * @return  Returns a pointer to the destination string.
+ */
 char *utoa(unsigned value, char *string, int radix)
 {
     return ultoa(value, string, radix);