Split out the description of page-level lock as new subsection in document.

Michael Banck

doc/src/sgml/mvcc.sgml

@@ -1139,6 +1139,10 @@ ERROR:  could not serialize access due to read/write dependencies among transact
      UPDATE</command> modifies selected rows to mark them locked, and so
      will result in disk writes.
     </para>
+   </sect2>
+
+   <sect2 id="locking-pages">
+    <title>Page-level Locks</title>
   
     <para>
      In addition to table and row locks, page-level share/exclusive locks are