sql/item_subselect.cc:
  Cleanup. Comments added.
sql/item_subselect.h:
  Cleanup.
sql/mysql_priv.h:
  Comments added.
sql/opt_subselect.cc:
  The function renamed and turned to method.
  Comments added.
sql/opt_subselect.h:
  The function turned to method of JOIN.
sql/sql_select.cc:
  Comment added. The function turned to method.
sql/sql_select.h:
  The function turned to method.