Add comment to AFL_* constants in Defines.php

$wgAntiLockFlags was removed in
Ia709ccdc9addc55c99cbff21a5ff3009b5fbb53c

Change-Id: Ia8685a84b72abb1a86b70f73c3f1e9f0c3e52a8f
Merge "Add comment to AFL_* constants in Defines.php"