Doc: add XML ID attributes to <sectN> and <varlistentry> tags.

This doesn't have any external effect at the moment, but it
will allow adding useful link-discoverability features later.

Brar Piening, reviewed by Karl Pinc.

Discussion: https://postgr.es/m/CAB8KJ=jpuQU9QJe4+RgWENrK5g9jhoysMw2nvTN_esoOU0=a_w@mail.gmail.com

doc/src/sgml/pgstatstatements.sgml

@@ -36,7 +36,7 @@
    <command>CREATE EXTENSION pg_stat_statements</command>.
  </para>
 
- <sect2>
+ <sect2 id="pgstatstatements-pg-stat-statements">
   <title>The <structname>pg_stat_statements</structname> View</title>
 
   <para>
@@ -602,7 +602,7 @@
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="pgstatstatements-pg-stat-statements-info">
   <title>The <structname>pg_stat_statements_info</structname> View</title>
 
   <indexterm>
@@ -658,7 +658,7 @@
   </table>
  </sect2>
 
- <sect2>
+ <sect2 id="pgstatstatements-funcs">
   <title>Functions</title>
 
   <variablelist>
@@ -721,7 +721,7 @@
   </variablelist>
  </sect2>
 
- <sect2>
+ <sect2 id="pgstatstatements-config-params">
   <title>Configuration Parameters</title>
 
   <variablelist>
@@ -856,7 +856,7 @@ pg_stat_statements.track = all
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="pgstatstatements-sample-output">
   <title>Sample Output</title>
 
 <screen>
@@ -961,7 +961,7 @@ hit_percent     |
 </screen>
  </sect2>
 
- <sect2>
+ <sect2 id="pgstatstatements-authors">
   <title>Authors</title>
 
   <para>