Provide initial module structure to Doxygen.

sql/sql_select.cc

@@ -13,6 +13,10 @@
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA */
 
+/**
+  @defgroup Query_Optimizer  Query Optimizer
+  @{
+*/
 
 /* mysql_select and join optimization */
 
@@ -16037,3 +16041,7 @@ bool JOIN::change_result(select_result *res)
   }
   DBUG_RETURN(FALSE);
 }
+
+/**
+  @} (end of group Query_Optimizer)
+*/