diff --git a/doc/src/sgml/catalogs.sgml b/doc/src/sgml/catalogs.sgml
index f1dcf8a..0a0c692 100644
--- a/doc/src/sgml/catalogs.sgml
+++ b/doc/src/sgml/catalogs.sgml
@@ -7193,7 +7193,9 @@
       <entry><literal><link linkend="catalog-pg-class"><structname>pg_class</structname></link>.oid</literal></entry>
       <entry>
        OID of the system catalog containing the lock target, or null if the
-       target is not a general database object
+       target is not a general database object.
+       For advisory locks it is the first int4 key
+       or the high half of the int8 key.
       </entry>
      </row>
      <row>
@@ -7203,8 +7205,8 @@
       <entry>
        OID of the lock target within its system catalog, or null if the
        target is not a general database object.
-       For advisory locks it is used to distinguish the two key
-       spaces (1 for an int8 key, 2 for two int4 keys).
+       For advisory locks it is the second int4 key
+       or the low half of the int8 key.
       </entry>
      </row>
      <row>
@@ -7216,7 +7218,9 @@
        <structfield>classid</> and <structfield>objid</> refer to the
        table itself),
        or zero if the target is some other general database object,
-       or null if the target is not a general database object
+       or null if the target is not a general database object.
+       For advisory locks it is used to distinguish the two key
+       spaces (1 for an int8 key, 2 for two int4 keys).
       </entry>
      </row>
      <row>