More updates and copy-editing. Rearrange order of sections a little bit

to put more widely useful info before less widely useful info.

doc/src/sgml/extend.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.28 2004/06/07 04:04:47 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.29 2004/12/30 03:13:56 tgl Exp $
 -->
 
  <chapter id="extend">
@@ -152,8 +152,8 @@ $PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.28 2004/06/07 04:04:47 tgl Exp $
 
     <para>
      Domains can be created using the <acronym>SQL</> command
-     <command>CREATE DOMAIN</command>.  Their creation and use is not
-     discussed in this chapter.
+     <xref linkend="sql-createdomain" endterm="sql-createdomain-title">.
+     Their creation and use is not discussed in this chapter.
     </para>
    </sect2>
 
@@ -221,7 +221,7 @@ $PostgreSQL: pgsql/doc/src/sgml/extend.sgml,v 1.28 2004/06/07 04:04:47 tgl Exp $
      Thus, when more than one argument position is declared with a polymorphic
      type, the net effect is that only certain combinations of actual argument
      types are allowed.  For example, a function declared as
-     <literal>foo(anyelement, anyelement)</> will take any two input values,
+     <literal>equal(anyelement, anyelement)</> will take any two input values,
      so long as they are of the same data type.
     </para>
 

doc/src/sgml/postgres.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/postgres.sgml,v 1.71 2004/12/29 23:36:47 tgl Exp $
+$PostgreSQL: pgsql/doc/src/sgml/postgres.sgml,v 1.72 2004/12/30 03:13:56 tgl Exp $
 -->
 
 <!DOCTYPE book PUBLIC "-//OASIS//DTD DocBook V4.2//EN" [
@@ -192,18 +192,19 @@ $PostgreSQL: pgsql/doc/src/sgml/postgres.sgml,v 1.71 2004/12/29 23:36:47 tgl Exp
     user-defined functions, data types, triggers, etc.  These are
     advanced topics which should probably be approached only after all
     the other user documentation about <productname>PostgreSQL</> has
-    been understood.  This part also describes the server-side
+    been understood.  Later chapters in this part describe the server-side
     programming languages available in the
     <productname>PostgreSQL</productname> distribution as well as
-    general issues concerning server-side programming languages.  This
-    information is only useful to readers that have read at least the
-    first few chapters of this part.
+    general issues concerning server-side programming languages.  It
+    is essential to read at least the earlier sections of <xref
+    linkend="extend"> (covering functions) before diving into the
+    material about server-side programming languages.
    </para>
   </partintro>
 
   &extend;
-  &rules;
   &trigger;
+  &rules;
 
   &xplang;
   &plsql;

doc/src/sgml/xplang.sgml

 <!--
-$PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.26 2003/11/29 19:51:38 pgsql Exp $
+$PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.27 2004/12/30 03:13:56 tgl Exp $
 -->
 
  <chapter id="xplang">
@@ -29,10 +29,16 @@ $PostgreSQL: pgsql/doc/src/sgml/xplang.sgml,v 1.26 2003/11/29 19:51:38 pgsql Exp
   <para>
    Writing a handler for a new procedural language is described in
    <xref linkend="plhandler">.  Several procedural languages are
-   available in the standard <productname>PostgreSQL</productname>
+   available in the core <productname>PostgreSQL</productname>
    distribution, which can serve as examples.
   </para>
 
+  <para>
+   There are additional procedural languages available that are not
+   included in the core distribution. <xref linkend="external-projects">
+   has information about finding them.
+  </para>
+
   <sect1 id="xplang-install">
    <title>Installing Procedural Languages</title>