PTR_DEMANGLE: add documentation

Signed-off-by: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>

libpthread/nptl/sysdeps/pthread/pthread-functions.h

@@ -103,7 +103,7 @@ struct pthread_functions
 extern struct pthread_functions __libc_pthread_functions attribute_hidden;
 extern int __libc_pthread_functions_init attribute_hidden;
 
-#if 0
+#if 0 /* def PTR_DEMANGLE */ /* we did not mangle, so do not demangle */
 # define PTHFCT_CALL(fct, params) \
   ({ __typeof (__libc_pthread_functions.fct) __p;			      \
      __p = __libc_pthread_functions.fct;				      \