Doc: improve documentation about ORDER BY in matviews.

Remove the confusing use of ORDER BY in an example materialized
view.  It adds nothing to the example, but might encourage
people to follow bad practice.  Clarify REFRESH MATERIALIZED
VIEW's note about whether view ordering is retained (it isn't).

Maciek Sakrejda

Discussion: https://postgr.es/m/CAOtHd0D-OvrUU0C=4hX28p4BaSE1XL78BAQ0VcDaLLt8tdUzsg@mail.gmail.com

doc/src/sgml/ref/refresh_materialized_view.sgml

@@ -93,12 +93,10 @@ REFRESH MATERIALIZED VIEW [ CONCURRENTLY ] <replaceable class="parameter">name</
   <title>Notes</title>
 
   <para>
-   While the default index for future
-   <link linkend="sql-cluster"><command>CLUSTER</command></link>
-   operations is retained, <command>REFRESH MATERIALIZED VIEW</command> does not
-   order the generated rows based on this property. If you want the data
-   to be ordered upon generation, you must use an <literal>ORDER BY</literal>
-   clause in the backing query.
+   If there is an <literal>ORDER BY</literal> clause in the materialized
+   view's defining query, the original contents of the materialized view
+   will be ordered that way; but <command>REFRESH MATERIALIZED
+   VIEW</command> does not guarantee to preserve that ordering.
   </para>
  </refsect1>
 

doc/src/sgml/rules.sgml

@@ -961,9 +961,6 @@ CREATE MATERIALIZED VIEW sales_summary AS
     FROM invoice
     WHERE invoice_date < CURRENT_DATE
     GROUP BY
-      seller_no,
-      invoice_date
-    ORDER BY
       seller_no,
       invoice_date;