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/ref/alter_foreign_table.sgml

@@ -72,7 +72,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       This form adds a new column to the foreign table, using the same syntax as
-      <xref linkend="sql-createforeigntable">.
+      <xref linkend="sql-createforeigntable"/>.
       Unlike the case when adding a column to a regular table, nothing happens
       to the underlying storage: this action simply declares that
       some new column is now accessible through the foreign table.
@@ -134,8 +134,8 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
      <para>
       This form
       sets the per-column statistics-gathering target for subsequent
-      <xref linkend="sql-analyze"> operations.
-      See the similar form of <xref linkend="sql-altertable">
+      <xref linkend="sql-analyze"/> operations.
+      See the similar form of <xref linkend="sql-altertable"/>
       for more details.
      </para>
     </listitem>
@@ -147,7 +147,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       This form sets or resets per-attribute options.
-      See the similar form of <xref linkend="sql-altertable">
+      See the similar form of <xref linkend="sql-altertable"/>
       for more details.
      </para>
     </listitem>
@@ -160,7 +160,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       This form sets the storage mode for a column.
-      See the similar form of <xref linkend="sql-altertable">
+      See the similar form of <xref linkend="sql-altertable"/>
       for more details.
       Note that the storage mode has no effect unless the table's
       foreign-data wrapper chooses to pay attention to it.
@@ -173,7 +173,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       This form adds a new constraint to a foreign table, using the same
-      syntax as <xref linkend="sql-createforeigntable">.
+      syntax as <xref linkend="sql-createforeigntable"/>.
       Currently only <literal>CHECK</literal> constraints are supported.
      </para>
 
@@ -182,7 +182,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
       done to verify the constraint is correct; rather, this action simply
       declares that some new condition should be assumed to hold for all rows
       in the foreign table.  (See the discussion
-      in <xref linkend="sql-createforeigntable">.)
+      in <xref linkend="sql-createforeigntable"/>.)
       If the constraint is marked <literal>NOT VALID</literal>, then it isn't
       assumed to hold, but is only recorded for possible future use.
      </para>
@@ -217,7 +217,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       These forms configure the firing of trigger(s) belonging to the foreign
-      table.  See the similar form of <xref linkend="sql-altertable"> for more
+      table.  See the similar form of <xref linkend="sql-altertable"/> for more
       details.
      </para>
     </listitem>
@@ -228,7 +228,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
     <listitem>
      <para>
       This form adds an <literal>oid</literal> system column to the
-      table (see <xref linkend="ddl-system-columns">).
+      table (see <xref linkend="ddl-system-columns"/>).
       It does nothing if the table already has OIDs.
       Unless the table's foreign-data wrapper supports OIDs, this column
       will simply read as zeroes.
@@ -261,7 +261,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
      <para>
       This form adds the target foreign table as a new child of the specified
       parent table.
-      See the similar form of <xref linkend="sql-altertable">
+      See the similar form of <xref linkend="sql-altertable"/>
       for more details.
      </para>
     </listitem>
@@ -433,7 +433,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
         Automatically drop objects that depend on the dropped column
         or constraint (for example, views referencing the column),
         and in turn all objects that depend on those objects
-        (see <xref linkend="ddl-depend">).
+        (see <xref linkend="ddl-depend"/>).
        </para>
       </listitem>
      </varlistentry>
@@ -525,7 +525,7 @@ ALTER FOREIGN TABLE [ IF EXISTS ] <replaceable class="parameter">name</replaceab
    </para>
 
    <para>
-    Refer to <xref linkend="sql-createforeigntable"> for a further description of valid
+    Refer to <xref linkend="sql-createforeigntable"/> for a further description of valid
     parameters.
    </para>
  </refsect1>
@@ -571,8 +571,8 @@ ALTER FOREIGN TABLE myschema.distributors OPTIONS (ADD opt1 'value', SET opt2 'v
   <title>See Also</title>
 
   <simplelist type="inline">
-   <member><xref linkend="sql-createforeigntable"></member>
-   <member><xref linkend="sql-dropforeigntable"></member>
+   <member><xref linkend="sql-createforeigntable"/></member>
+   <member><xref linkend="sql-dropforeigntable"/></member>
   </simplelist>
  </refsect1>
 </refentry>