docs: add xreflabel entries for autovacuum, SP-GiST, and TOAST

This is for use by the PG 13 release notes, but might be used for minor
release notes in the future.

Backpatch-through: 9.5

doc/src/sgml/maintenance.sgml

@@ -707,7 +707,7 @@ HINT:  Stop the postmaster and vacuum that database in single-user mode.
    </sect3>
   </sect2>
 
-  <sect2 id="autovacuum">
+  <sect2 id="autovacuum" xreflabel="autovacuum">
    <title>The Autovacuum Daemon</title>
 
    <indexterm>

doc/src/sgml/spgist.sgml

 <!-- doc/src/sgml/spgist.sgml -->
 
-<chapter id="spgist">
+<chapter id="spgist" xreflabel="SP-GiST">
 <title>SP-GiST Indexes</title>
 
    <indexterm>

doc/src/sgml/storage.sgml

@@ -301,7 +301,7 @@ where <replaceable>PPP</replaceable> is the PID of the owning backend and
 
 </sect1>
 
-<sect1 id="storage-toast">
+<sect1 id="storage-toast" xreflabel="TOAST">
 
 <title>TOAST</title>