Fix harmless warnings with mingw in timing.c
diff --git a/include/polarssl/timing.h b/include/polarssl/timing.h
index a3eb510..5f3acfa 100644
--- a/include/polarssl/timing.h
+++ b/include/polarssl/timing.h
@@ -65,6 +65,10 @@
  * \brief          Setup an alarm clock
  *
  * \param seconds  delay before the "alarmed" flag is set
+ *
+ * \warning        Only one alarm at a time  is supported. In a threaded
+ *                 context, this means one for the whole process, not one per
+ *                 thread.
  */
 void set_alarm( int seconds );