Modify the definition of window-function PARTITION BY and ORDER BY clauses

so that their elements are always taken as simple expressions over the
query's input columns.  It originally seemed like a good idea to make them
act exactly like GROUP BY and ORDER BY, right down to the SQL92-era behavior
of accepting output column names or numbers.  However, that was not such a
great idea, for two reasons:

1. It permits circular references, as exhibited in bug #5018: the output
column could be the one containing the window function itself.  (We actually
had a regression test case illustrating this, but nobody thought twice about
how confusing that would be.)

2. It doesn't seem like a good idea for, eg, "lead(foo) OVER (ORDER BY foo)"
to potentially use two completely different meanings for "foo".

Accordingly, narrow down the behavior of window clauses to use only the
SQL99-compliant interpretation that the expressions are simple expressions.

doc/src/sgml/syntax.sgml

@@ -1,4 +1,4 @@
-<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.133 2009/06/17 21:58:49 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/syntax.sgml,v 1.134 2009/08/27 20:08:02 tgl Exp $ -->
 
 <chapter id="sql-syntax">
  <title>SQL Syntax</title>
@@ -1619,7 +1619,9 @@ ROWS BETWEEN UNBOUNDED PRECEDING AND UNBOUNDED FOLLOWING
     expression that does not itself contain window function calls.
     The <literal>PARTITION BY</> and <literal>ORDER BY</> lists have
     essentially the same syntax and semantics as <literal>GROUP BY</>
-    and <literal>ORDER BY</> clauses of the whole query.
+    and <literal>ORDER BY</> clauses of the whole query, except that their
+    expressions are always just expressions and cannot be output-column
+    names or numbers.
     <replaceable>window_name</replaceable> is a reference to a named window
     specification defined in the query's <literal>WINDOW</literal> clause.
     Named window specifications are usually referenced with just