doc: Mention UNION/ORDER BY etc. keywords in section headers.

Most of the section and sub-section headers in the Queries chapter have
the keywords literally stated, but neither "Sorting Rows" nor "Combining
Rows" did. There's no rule that they must be, but it seems like a good
practice. The keywords will ring a bell to anyone with with even a little
bit of SQL experience.

David G. Johnston, per suggestion by bilge@scriptfusion.com

Discussion: https://www.postgresql.org/message-id/159981394174.31338.7014519396749859167%40wrigleys.postgresql.org

doc/src/sgml/queries.sgml

@@ -1621,7 +1621,7 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
 
 
  <sect1 id="queries-union">
-  <title>Combining Queries</title>
+  <title>Combining Queries (<literal>UNION</literal>, <literal>INTERSECT</literal>, <literal>EXCEPT</literal>)</title>
 
   <indexterm zone="queries-union">
    <primary>UNION</primary>
@@ -1701,7 +1701,7 @@ SELECT DISTINCT ON (<replaceable>expression</replaceable> <optional>, <replaceab
 
 
  <sect1 id="queries-order">
-  <title>Sorting Rows</title>
+  <title>Sorting Rows (<literal>ORDER BY</literal>)</title>
 
   <indexterm zone="queries-order">
    <primary>sorting</primary>