[3752] Added a precision about (immediate) flush (true)

doc/guide/logging.xml

@@ -542,7 +542,7 @@ TODO; there's a ticket to determine these levels, see #1074
             Flush buffers after each log message. Doing this will
             reduce performance but will ensure that if the program
             terminates abnormally, all messages up to the point of
-            termination are output.
+            termination are output. Default is true.
           </para>
 
         </section>