Restructure foreign data wrapper chapter so it has more than one section.

As noted by Laurenz Albe, our SGML tools deal rather oddly with chapters
having just one <sect1>.  Perhaps the tooling could be fixed, but really
the design of this chapter's introduction is pretty bogus anyhow.  Split
it into a true introduction and a <sect1> about the FDW functions, so
that it reads better and dodges the lack-of-a-chapter-TOC problem.

doc/src/sgml/fdwhandler.sgml

@@ -17,6 +17,26 @@
     to write a new foreign data wrapper.
    </para>
 
+   <para>
+    The foreign data wrappers included in the standard distribution are good
+    references when trying to write your own.  Look into the
+    <filename>contrib/file_fdw</> subdirectory of the source tree.
+    The <xref linkend="sql-createforeigndatawrapper"> reference page also has
+    some useful details.
+   </para>
+
+   <note>
+    <para>
+     The SQL standard specifies an interface for writing foreign data wrappers.
+     However, PostgreSQL does not implement that API, because the effort to
+     accommodate it into PostgreSQL would be large, and the standard API hasn't
+     gained wide adoption anyway.
+    </para>
+   </note>
+
+   <sect1 id="fdw-functions">
+    <title>Foreign Data Wrapper Functions</title>
+
     <para>
      The FDW author needs to implement a handler function, and optionally
      a validator function. Both functions must be written in a compiled
@@ -31,10 +51,11 @@
      Most of the effort in writing an FDW is in implementing these callback
      functions.
      The handler function must be registered with
-    <productname>PostgreSQL</productname> as taking no arguments and returning
-    the special pseudo-type <type>fdw_handler</type>.
-    The callback functions are plain C functions and are not visible or
-    callable at the SQL level.
+     <productname>PostgreSQL</productname> as taking no arguments and
+     returning the special pseudo-type <type>fdw_handler</type>.  The
+     callback functions are plain C functions and are not visible or
+     callable at the SQL level.  The callback functions are described in
+     <xref linkend="fdw-callbacks">.
     </para>
 
     <para>
@@ -42,10 +63,11 @@
      <command>CREATE</command> and <command>ALTER</command> commands for its
      foreign data wrapper, as well as foreign servers, user mappings, and
      foreign tables using the wrapper.
-    The validator function must be registered as taking two arguments, a text
-    array containing the options to be validated, and an OID representing the
-    type of object the options are associated with (in the form of the OID
-    of the system catalog the object would be stored in, either
+     The validator function must be registered as taking two arguments, a
+     text array containing the options to be validated, and an OID
+     representing the type of object the options are associated with (in
+     the form of the OID of the system catalog the object would be stored
+     in, either
      <literal>ForeignDataWrapperRelationId</>,
      <literal>ForeignServerRelationId</>,
      <literal>UserMappingRelationId</>,
@@ -54,24 +76,9 @@
      creation time or object alteration time.
     </para>
 
-   <para>
-    The foreign data wrappers included in the standard distribution are good
-    references when trying to write your own.  Look into the
-    <filename>contrib/file_fdw</> subdirectory of the source tree.
-    The <xref linkend="sql-createforeigndatawrapper"> reference page also has
-    some useful details.
-   </para>
-
-   <note>
-    <para>
-     The SQL standard specifies an interface for writing foreign data wrappers.
-     However, PostgreSQL does not implement that API, because the effort to
-     accommodate it into PostgreSQL would be large, and the standard API hasn't
-     gained wide adoption anyway.
-    </para>
-   </note>
+   </sect1>
 
-   <sect1 id="fdw-routines">
+   <sect1 id="fdw-callbacks">
     <title>Foreign Data Wrapper Callback Routines</title>
 
     <para>