Clarified "not allowing column alias in WHERE clause" explicitly in SELECT section.

Docs/manual.texi

@@ -31604,6 +31604,12 @@ mysql> select concat(last_name,', ',first_name) AS full_name
     from mytable ORDER BY full_name;
 @end example
 
+@item
+It is not allowed to use a column alias in a @code{WHERE} clause,
+because the column value may not yet be determined when the
+@code{WHERE} clause is executed.
+@xref{Problems with alias}.
+
 @item
 The @code{FROM table_references} clause indicates the tables from which to
 retrieve rows.  If you name more than one table, you are performing a