Added a comment

--HG--
extra : convert_revision : svn%3Afdd8eb12-d10e-0410-9acb-85c331704f74/trunk%403716

examples/thread_function_ex.cpp

@@ -58,6 +58,7 @@ int main()
     double val = 2;
     thread_function t4(thread_increment, ref(val));
     t4.wait(); // wait for t4 to finish before printing val.
+    // Print val.  It will now have a value of 3.
     cout << "val: " << val << endl;