add and amend comments for clarity

include/my_base.h

@@ -170,9 +170,8 @@ enum ha_extra_function {
   HA_EXTRA_DELETE_CANNOT_BATCH,
   HA_EXTRA_UPDATE_CANNOT_BATCH,
   /*
-    Inform handler that write_row() should immediately report constraint
-    violations because a INSERT...ON DUPLICATE KEY UPDATE is in being
-    performed.
+    Inform handler that an "INSERT...ON DUPLICATE KEY UPDATE" will be
+    executed. This condition is unset by HA_EXTRA_NO_IGNORE_DUP_KEY.
   */
   HA_EXTRA_INSERT_WITH_UPDATE
 };

sql/ha_federated.cc

@@ -2555,6 +2555,10 @@ int ha_federated::extra(ha_extra_function operation)
     replace_duplicates= TRUE;
     break;
   case HA_EXTRA_WRITE_CANNOT_REPLACE:
+    /*
+      We use this flag to ensure that we do not create an "INSERT IGNORE"
+      statement when inserting new rows into the remote table.
+    */
     replace_duplicates= FALSE;
     break;
   case HA_EXTRA_INSERT_WITH_UPDATE: