[2202] More detail in TODO comment

src/bin/auth/tests/auth_srv_unittest.cc

@@ -1802,7 +1802,9 @@ TEST_F(AuthSrvTest, clientList) {
 // We just test the mutex can be locked (exactly once).
 TEST_F(AuthSrvTest, mutex) {
     isc::util::thread::Mutex::Locker l1(server.getClientListMutex());
-    // TODO: Once we have non-debug build, this one will not work.
+    // TODO: Once we have non-debug build, this one will not work, since
+    // we currently use the fact that we can't lock twice from the same
+    // thread. In the non-debug mode, this would deadlock.
     // Skip then.
     EXPECT_THROW({
         isc::util::thread::Mutex::Locker l2(server.getClientListMutex());