Remove erroneous claim about use of pg_locks.objid for advisory locks.

The correct information appears in the text, so just remove the statement
in the table, where it did not fit nicely anyway.  (Curiously, the correct
info has been there much longer than the erroneous table entry.)
Resolves problem noted by Daniele Varrazzo.

In HEAD and 9.1, also do a bit of wordsmithing on other text on the page.

doc/src/sgml/catalogs.sgml

@@ -5990,10 +5990,7 @@
       <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.
-       For advisory locks it is used to distinguish the two key
-       spaces (<literal>1</> for an int8 key, <literal>2</> for two
-       int4 keys).
+       object is not a general database object
       </entry>
      </row>
      <row>