core: documentation: document msg_reply_int()

core/include/msg.h

@@ -207,6 +207,20 @@ int msg_send_receive(msg_t *m, msg_t *reply, kernel_pid_t target_pid);
  */
 int msg_reply(msg_t *m, msg_t *reply);
 
+/**
+ * @brief Replies to a message from interrupt.
+ *
+ * An ISR can obviously not receive messages, however a thread might delegate
+ * replying to a message to an ISR.
+ *
+ * @param[in] m         message to reply to, must not be NULL.
+ * @param[out] reply    message that target will get as reply, must not be NULL.
+ *
+ * @return 1, if successful
+ * @return -1, on error
+ */
+int msg_reply_int(msg_t *m, msg_t *reply)
+
 /**
  * @brief Initialize the current thread's message queue.
  *