Document use of pg_locks.objid for advisory locks, suggestion from Marc Mamin

doc/src/sgml/catalogs.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.161 2008/01/31 18:40:02 tgl Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/catalogs.sgml,v 2.162 2008/03/06 18:49:32 momjian Exp $ -->
 <!--
  Documentation of the system catalogs, directed toward PostgreSQL developers
  -->
@@ -5691,7 +5691,10 @@
       <entry>any OID column</entry>
       <entry>
        OID of the object within its system catalog, or NULL if the
-       object is not a general database object
+       object is not a general database object.
+       For advisory locks it is used to distinguish the two key
+       spaces (<literal>1</> for an int8 key, <literal>2</> for two
+       int4 keys).
       </entry>
      </row>
      <row>

doc/src/sgml/func.sgml

-<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.422 2008/03/03 18:09:02 momjian Exp $ -->
+<!-- $PostgreSQL: pgsql/doc/src/sgml/func.sgml,v 1.423 2008/03/06 18:49:32 momjian Exp $ -->
 
  <chapter id="functions">
   <title>Functions and Operators</title>
@@ -12378,7 +12378,8 @@ SELECT (pg_stat_file('filename')).modification;
    <para>
     <function>pg_advisory_lock</> locks an application-defined resource,
     which can be identified either by a single 64-bit key value or two
-    32-bit key values (note that these two key spaces do not overlap).  If
+    32-bit key values (note that these two key spaces do not overlap). 
+    The key type is specified in <literal>pg_locks.objid</>.  If
     another session already holds a lock on the same resource, the
     function will wait until the resource becomes available.  The lock
     is exclusive.  Multiple lock requests stack, so that if the same resource