doc: Correct description of restart_lsn in pg_replication_slots

Previously the document explained that restart_lsn indicates the LSN of
oldest WAL won't be automatically removed during checkpoints. But
since v13 this was no longer true thanks to max_slot_wal_keep_size.

Back-patch to v13 where max_slot_wal_keep_size was added.

Author: Fujii Masao
Discussion: https://postgr.es/m/6497f1e9-3148-c5da-7e49-b2fddad9a42f@oss.nttdata.com

doc/src/sgml/catalogs.sgml

@@ -11214,7 +11214,9 @@ SELECT * FROM pg_locks pl LEFT JOIN pg_prepared_xacts ppx
       <para>
        The address (<literal>LSN</literal>) of oldest WAL which still
        might be required by the consumer of this slot and thus won't be
-       automatically removed during checkpoints.  <literal>NULL</literal>
+       automatically removed during checkpoints unless this LSN
+       gets behind more than <xref linkend="guc-max-slot-wal-keep-size"/>
+       from the current LSN.  <literal>NULL</literal>
        if the <literal>LSN</literal> of this slot has never been reserved.
       </para></entry>
      </row>