Document how to create advisory lock "bigint" values in SQL.

David E. Wheeler

doc/src/sgml/catalogs.sgml

@@ -7412,7 +7412,10 @@
    A <type>bigint</type> key is displayed with its
    high-order half in the <structfield>classid</> column, its low-order half
    in the <structfield>objid</> column, and <structfield>objsubid</> equal
-   to 1.  Integer keys are displayed with the first key in the
+   to 1. The original <type>bigint</type> value can be reassembled with the
+   expression <literal>(classid::int::bit(64) &lt;&lt; 32 |
+   objid::int::bit(64))::bigint</literal>. Integer keys are displayed with the
+   first key in the
    <structfield>classid</> column, the second key in the <structfield>objid</>
    column, and <structfield>objsubid</> equal to 2.  The actual meaning of
    the keys is up to the user.  Advisory locks are local to each database,