Doc: update documentation for requirement of ORDER BY in GROUPS mode.

Commit ff4f88916 adjusted the code to enforce the SQL spec's requirement
that a window using GROUPS mode must have an ORDER BY clause.  But I missed
that the documentation explicitly said you didn't have to have one.

Also minor wordsmithing in the window-function section of select.sgml.

Per Masahiko Sawada, though I didn't use his patch.

doc/src/sgml/ref/select.sgml

@@ -901,8 +901,8 @@ EXCLUDE NO OTHERS
     CURRENT ROW</literal>; it sets the frame to be all rows from the partition start
     up through the current row's last <firstterm>peer</firstterm> (a row
     that the window's <literal>ORDER BY</literal> clause considers
-    equivalent to the current row), or all rows if there
-    is no <literal>ORDER BY</literal>.
+    equivalent to the current row; all rows are peers if there
+    is no <literal>ORDER BY</literal>).
     In general, <literal>UNBOUNDED PRECEDING</literal> means that the frame
     starts with the first row of the partition, and similarly
     <literal>UNBOUNDED FOLLOWING</literal> means that the frame ends with the last
@@ -924,7 +924,7 @@ EXCLUDE NO OTHERS
     is an integer indicating that the frame starts or ends that many peer
     groups before or after the current row's peer group, where
     a <firstterm>peer group</firstterm> is a group of rows that are
-    equivalent according to <literal>ORDER BY</literal>.
+    equivalent according to the window's <literal>ORDER BY</literal> clause.
     In <literal>RANGE</literal> mode, use of
     an <replaceable>offset</replaceable> option requires that there be
     exactly one <literal>ORDER BY</literal> column in the window definition.

doc/src/sgml/syntax.sgml

@@ -1917,8 +1917,8 @@ EXCLUDE NO OTHERS
        ends the specified number of <firstterm>peer groups</firstterm>
        before or after the current row's peer group, where a peer group is a
        set of rows that are equivalent in the <literal>ORDER BY</literal>
-       ordering.  (If there is no <literal>ORDER BY</literal>, the whole
-       partition is one peer group.)
+       ordering.  (There must be an <literal>ORDER BY</literal> clause
+       in the window definition to use <literal>GROUPS</literal> mode.)
       </para>
      </listitem>
      <listitem>