Correct overstatement in locking docs: we said ExclusiveLock is never

taken automatically, but this is only true with respect to user tables.

doc/src/sgml/mvcc.sgml

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  <chapter id="mvcc">
@@ -677,8 +677,9 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
 	</para>
 
 	<para>
-         This lock mode is not automatically acquired by any
-         <productname>PostgreSQL</productname> command.
+         This lock mode is not automatically acquired on user tables by any
+         <productname>PostgreSQL</productname> command.  However it is
+         acquired on certain system catalogs in some operations.
 	</para>
        </listitem>
       </varlistentry>