diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index 700271fd40..bf326cf533 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/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>