Doc: fix bogus cross-reference link.

An xref to a <para>'s ID isn't very helpful because paras don't have
names.  This causes a warning while building PDFs, though for some
reason not while building HTML.  The link arguably went to the wrong
place, too.

To fix, introduce a sub-section we can reference.

doc/src/sgml/jit.sgml

@@ -58,9 +58,9 @@
    </para>
    <para>
     Tuple deforming is the process of transforming an on-disk tuple (see <xref
-    linkend="heaptuple"/>) into its in-memory representation. It can be
-    accelerated by creating a function specific to the table layout and the
-    number of columns to be extracted.
+    linkend="storage-tuple-layout"/>) into its in-memory representation.
+    It can be accelerated by creating a function specific to the table layout
+    and the number of columns to be extracted.
    </para>
   </sect2>
 

doc/src/sgml/storage.sgml

@@ -875,8 +875,7 @@ data. Empty in ordinary tables.</entry>
   <filename>src/include/storage/bufpage.h</filename>.
  </para>
 
- <para id="heaptuple">
-
+ <para>
   Following the page header are item identifiers
   (<type>ItemIdData</type>), each requiring four bytes.
   An item identifier contains a byte-offset to
@@ -917,6 +916,10 @@ data. Empty in ordinary tables.</entry>
 
  </para>
 
+ <sect2 id="storage-tuple-layout">
+
+ <title>Table Row Layout</title>
+
  <para>
 
   All table rows are structured in the same way. There is a fixed-size
@@ -1040,6 +1043,7 @@ data. Empty in ordinary tables.</entry>
   it might be compressed, too (see <xref linkend="storage-toast"/>).
 
  </para>
+ </sect2>
 </sect1>
 
 </chapter>