Add more parallel query documentation.

Previously, the individual settings were documented, but there was
no overall discussion of the capabilities and limitations of the
feature.  Add that.

Patch by me, reviewed by Peter Eisentraut and Álvaro Herrera.

doc/src/sgml/config.sgml

@@ -2027,6 +2027,11 @@ include_dir 'conf.d'
          as much CPU time, memory, I/O bandwidth, and so forth as a query which
          uses no workers at all.
         </para>
+
+        <para>
+         For more information on parallel query, see
+         <xref linkend="parallel-query">.
+        </para>
        </listitem>
       </varlistentry>
 

doc/src/sgml/filelist.sgml

@@ -24,6 +24,7 @@
 <!ENTITY indices    SYSTEM "indices.sgml">
 <!ENTITY json       SYSTEM "json.sgml">
 <!ENTITY mvcc       SYSTEM "mvcc.sgml">
+<!ENTITY parallel   SYSTEM "parallel.sgml">
 <!ENTITY perform    SYSTEM "perform.sgml">
 <!ENTITY queries    SYSTEM "queries.sgml">
 <!ENTITY rangetypes SYSTEM "rangetypes.sgml">

doc/src/sgml/postgres.sgml

@@ -106,6 +106,7 @@
   &textsearch;
   &mvcc;
   &perform;
+  ∥
 
  </part>