Convert documentation to DocBook XML

Since some preparation work had already been done, the only source
changes left were changing empty-element tags like <xref linkend="foo">
to <xref linkend="foo"/>, and changing the DOCTYPE.

The source files are still named *.sgml, but they are actually XML files
now.  Renaming could be considered later.

In the build system, the intermediate step to convert from SGML to XML
is removed.  Everything is build straight from the source files again.
The OpenSP (or the old SP) package is no longer needed.

The documentation toolchain instructions are updated and are much
simpler now.

Peter Eisentraut, Alexander Lakhin, Jürgen Purtz

doc/src/sgml/gin.sgml

@@ -68,8 +68,8 @@
  <para>
   The core <productname>PostgreSQL</productname> distribution
   includes the <acronym>GIN</acronym> operator classes shown in
-  <xref linkend="gin-builtin-opclasses-table">.
-  (Some of the optional modules described in <xref linkend="contrib">
+  <xref linkend="gin-builtin-opclasses-table"/>.
+  (Some of the optional modules described in <xref linkend="contrib"/>
   provide additional <acronym>GIN</acronym> operator classes.)
  </para>
 
@@ -127,7 +127,7 @@
   Of the two operator classes for type <type>jsonb</type>, <literal>jsonb_ops</literal>
   is the default.  <literal>jsonb_path_ops</literal> supports fewer operators but
   offers better performance for those operators.
-  See <xref linkend="json-indexing"> for details.
+  See <xref linkend="json-indexing"/> for details.
  </para>
 
 </sect1>
@@ -182,7 +182,7 @@
        <literal>query</literal> is the value on the right-hand side of an
        indexable operator whose left-hand side is the indexed column.
        <literal>n</literal> is the strategy number of the operator within the
-       operator class (see <xref linkend="xindex-strategies">).
+       operator class (see <xref linkend="xindex-strategies"/>).
        Often, <function>extractQuery</function> will need
        to consult <literal>n</literal> to determine the data type of
        <literal>query</literal> and the method it should use to extract key values.
@@ -406,7 +406,7 @@
   provide the <function>comparePartial</function> method, and its
   <function>extractQuery</function> method must set the <literal>pmatch</literal>
   parameter when a partial-match query is encountered.  See
-  <xref linkend="gin-partial-match"> for details.
+  <xref linkend="gin-partial-match"/> for details.
  </para>
 
  <para>
@@ -466,7 +466,7 @@
    When the table is vacuumed or autoanalyzed, or when
    <function>gin_clean_pending_list</function> function is called, or if the
    pending list becomes larger than
-   <xref linkend="guc-gin-pending-list-limit">, the entries are moved to the
+   <xref linkend="guc-gin-pending-list-limit"/>, the entries are moved to the
    main <acronym>GIN</acronym> data structure using the same bulk insert
    techniques used during initial index creation.  This greatly improves
    <acronym>GIN</acronym> index update speed, even counting the additional
@@ -488,7 +488,7 @@
    If consistent response time is more important than update speed,
    use of pending entries can be disabled by turning off the
    <literal>fastupdate</literal> storage parameter for a
-   <acronym>GIN</acronym> index.  See <xref linkend="sql-createindex">
+   <acronym>GIN</acronym> index.  See <xref linkend="sql-createindex"/>
    for details.
   </para>
  </sect2>
@@ -531,14 +531,14 @@
     <para>
      As of <productname>PostgreSQL</productname> 8.4, this advice is less
      necessary since delayed indexing is used (see <xref
-     linkend="gin-fast-update"> for details).  But for very large updates
+     linkend="gin-fast-update"/> for details).  But for very large updates
      it may still be best to drop and recreate the index.
     </para>
    </listitem>
   </varlistentry>
 
   <varlistentry>
-   <term><xref linkend="guc-maintenance-work-mem"></term>
+   <term><xref linkend="guc-maintenance-work-mem"/></term>
    <listitem>
     <para>
      Build time for a <acronym>GIN</acronym> index is very sensitive to
@@ -549,7 +549,7 @@
   </varlistentry>
 
   <varlistentry>
-   <term><xref linkend="guc-gin-pending-list-limit"></term>
+   <term><xref linkend="guc-gin-pending-list-limit"/></term>
    <listitem>
     <para>
      During a series of insertions into an existing <acronym>GIN</acronym>
@@ -574,7 +574,7 @@
   </varlistentry>
 
   <varlistentry>
-   <term><xref linkend="guc-gin-fuzzy-search-limit"></term>
+   <term><xref linkend="guc-gin-fuzzy-search-limit"/></term>
    <listitem>
     <para>
      The primary goal of developing <acronym>GIN</acronym> indexes was
@@ -631,7 +631,7 @@
  <para>
   The core <productname>PostgreSQL</productname> distribution
   includes the <acronym>GIN</acronym> operator classes previously shown in
-  <xref linkend="gin-builtin-opclasses-table">.
+  <xref linkend="gin-builtin-opclasses-table"/>.
   The following <filename>contrib</filename> modules also contain
   <acronym>GIN</acronym> operator classes: