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/intarray.sgml

@@ -30,7 +30,7 @@
   on the current database.
  </para>
 
- <sect2>
+ <sect2 id="intarray-funcs-ops">
   <title><filename>intarray</filename> Functions and Operators</title>
 
   <para>
@@ -391,7 +391,7 @@
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="intarray-index">
   <title>Index Support</title>
 
   <para>
@@ -443,7 +443,7 @@
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="intarray-example">
   <title>Example</title>
 
 <programlisting>
@@ -464,7 +464,7 @@ SELECT message.mid FROM message WHERE message.sections @@ '1&amp;2'::query_int;
 </programlisting>
  </sect2>
 
- <sect2>
+ <sect2 id="intarray-benchmark">
   <title>Benchmark</title>
 
   <para>
@@ -488,7 +488,7 @@ psql -c "CREATE EXTENSION intarray" TEST
   </para>
  </sect2>
 
- <sect2>
+ <sect2 id="intarray-Authors">
   <title>Authors</title>
 
   <para>