Improve a comment.

    This is to track items changed during execution of a prepared

    statement/stored procedure. It's created by

    register_item_tree_change() in memory root of THD, and freed in

    rollback_item_tree_changes(). For conventional execution it's always 0.

    rollback_item_tree_changes(). For conventional execution it's always

    empty.

  */

  Item_change_list change_list;

  /*

    Current prepared Query_arena if there one, or 0

    A permanent memory area of the statement. For conventional

    execution, the parsed tree and execution runtime reside in the same

    memory root. In this case current_arena points to THD. In case of

    a prepared statement or a stored procedure statement, thd->mem_root

    conventionally points to runtime memory, and thd->current_arena

    points to the memory of the PS/SP, where the parsed tree of the

    statement resides. Whenever you need to perform a permanent

    transformation of a parsed tree, you should allocate new memory in

    current_arena, to allow correct re-execution of PS/SP.

    Note: in the parser, current_arena == thd, even for PS/SP.

  */

  Query_arena *current_arena;

  /*