Clarify pg_rewind connection requirements.

Per http://www.postgresql.org/message-id/flat/564C4CE6.9000509@postgrespro.ru
Pavel Luzanov <p.luzanov@postgrespro.ru>

doc/src/sgml/ref/pg_rewind.sgml

@@ -130,7 +130,9 @@ PostgreSQL documentation
        <para>
         Specifies a libpq connection string to connect to the source
         <productname>PostgreSQL</> server to synchronize the target with.
-        The server must be up and running, and must not be in recovery mode.
+        The connection must be a normal (non-replication) connection 
+        with superuser access. The server must be up and running, 
+        and must not be in recovery mode.
        </para>
       </listitem>
      </varlistentry>