Improve SELECT FOR UPDATE documentation and remove outdated comment

h2/src/docsrc/help/help.csv

@@ -74,6 +74,8 @@ If FOR UPDATE is specified, the tables or rows are locked for writing.
 This clause is not allowed in DISTINCT queries and in queries with non-window aggregates, GROUP BY, or HAVING clauses.
 When using default MVStore engine only the selected rows are locked as in an UPDATE statement;
 locking behavior for rows that were excluded from result using OFFSET / FETCH or QUALIFY is undefined.
+Rows are processed one by one. Committed row is read, tested with WHERE criteria, locked, read again and re-tested,
+because its value may be changed by concurrent transaction before lock acquisition.
 With PageStore engine the whole tables are locked.
 ","
 SELECT * FROM TEST;

h2/src/main/org/h2/table/TableFilter.java

@@ -606,8 +606,6 @@ public class TableFilter implements ColumnResolver {
      * @param current the current row
      */
     public void set(Row current) {
-        // this is currently only used so that check constraints work - to set
-        // the current (new) row
         this.current = current;
         this.currentSearchRow = current;
     }