Remove old claim that ExclusiveLock is sometimes taken on system catalogs.

We used to do that on pg_listener, but pg_listener is no more.

Also add a bit more documentation for ShareRowExclusive mode.

doc/src/sgml/mvcc.sgml

@@ -692,6 +692,8 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
          <literal>SHARE</literal>, <literal>SHARE ROW
          EXCLUSIVE</literal>, <literal>EXCLUSIVE</literal>, and
          <literal>ACCESS EXCLUSIVE</literal> lock modes.
+         This mode protects a table against concurrent data changes, and
+         is self-exclusive so that only one session can hold it at a time.
         </para>
 
         <para>
@@ -719,9 +721,8 @@ SELECT SUM(value) FROM mytab WHERE class = 2;
         </para>
 
         <para>
-         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.
+         This lock mode is not automatically acquired on tables by any
+         <productname>PostgreSQL</productname> command.
         </para>
        </listitem>
       </varlistentry>