Added \deprecated to exit_thread() function.

Reflects Guido's revision.

Doc/lib/libthread.tex

@@ -44,6 +44,7 @@ will cause the thread to exit silently.
 \end{funcdesc}
 
 \begin{funcdesc}{exit_thread}{}
+\deprecated{1.5.2}{Use \function{exit()}.}
 This is an obsolete synonym for \function{exit()}.
 \end{funcdesc}