Add a note to the MVCC chapter that some things aren't transactional.

Craig Ringer, slightly edited by me.

doc/src/sgml/mvcc.sgml

@@ -260,6 +260,18 @@
     command <xref linkend="sql-set-transaction">.
    </para>
 
+   <important>
+     <para>
+       Some <productname>PostgreSQL</productname> data types and functions have
+       special rules regarding transactional behaviour.  In particular, changes
+       made to a <literal>SEQUENCE</literal> (and therefore the counter of a
+       column declared using <literal>SERIAL</literal>) are immediately visible
+       to all other transactions and are not rolled back if the transaction
+       that made the changes aborts.  See <xref linkend="functions-sequence">
+       and <xref linkend="datatype-serial">.
+     </para>
+   </important>
+
   <sect2 id="xact-read-committed">
    <title>Read Committed Isolation Level</title>