Made the specs more clear.

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%402829

dlib/gui_widgets/base_widgets_abstract.h

@@ -28,7 +28,10 @@ namespace dlib
             be able to process the event. 
 
             Also note that all event handlers, including the user registered callback
-            functions, are executed in the event handling thread. 
+            functions, are executed in the event handling thread.   Additionally,
+            the drawable::m mutex will always be locked while these event handlers
+            are running.  Also, don't rely on get_thread_id() always returning the 
+            same ID from inside event handlers.
     !*/
 
 // ----------------------------------------------------------------------------------------

dlib/gui_widgets/widgets_abstract.h

@@ -32,7 +32,10 @@ namespace dlib
 
         EVENT HANDLERS
             Note that all event handlers, including the user registered callback
-            functions, are executed in the event handling thread. 
+            functions, are executed in the event handling thread.   Additionally,
+            the drawable::m mutex will always be locked while these event handlers
+            are running.  Also, don't rely on get_thread_id() always returning the 
+            same ID from inside event handlers.
     !*/
 
 // ----------------------------------------------------------------------------------------