docs(core): Add help description for RT_USING_THREADSAFE_PRINTF

src/Kconfig

@@ -413,8 +413,15 @@ config RT_USING_INTERRUPT_INFO
         Add name and counter information for interrupt trace.
 
 config RT_USING_THREADSAFE_PRINTF
-    bool "Enable thread safe kernel print service"
+    bool "Enable thread-safe kernel print service (not recommended for hard real-time)"
     default y if RT_USING_SMP && RT_USING_SMART
+    help
+        Note : this option may increase worst-case
+        interrupt/scheduling latency, because console output is protected in
+        a non-preemptible section and device write can be relatively slow.
+        If strict real-time response is required, avoid frequent synchronous
+        kprintf output in time-critical paths, or prefer asynchronous logging
+        (for example ULog async mode).
 
 config RT_USING_CONSOLE
     bool "Using console for rt_kprintf"