ddl: address comment.

ddl/column.go

@@ -329,6 +329,12 @@ func (d *ddl) onDropColumn(t *meta.Meta, job *model.Job) error {
 	}
 }
 
+// How to backfill column data in reorganization state?
+//  1, Generate a snapshot with special version.
+//  2, Traverse the snapshot, get every row in the table.
+//  3, For one row, if the row has been already deleted, skip to next row.
+//  4, If not deleted, check whether column data has existed, if existed, skip to next row.
+//  5, If column data doesn't exist, backfill the column with default value and then continue to handle next row.
 func (d *ddl) backfillColumn(t table.Table, columnInfo *model.ColumnInfo, version uint64, seekHandle int64) error {
 	for {
 		handles, err := d.getSnapshotRows(t, version, seekHandle)