Clarify vacuum lazy can shrink a file under certain circumstances.

doc/src/sgml/maintenance.sgml

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+$PostgreSQL: pgsql/doc/src/sgml/maintenance.sgml,v 1.52 2005/12/07 05:35:53 momjian Exp $
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 <chapter id="maintenance">
@@ -141,11 +141,12 @@ $PostgreSQL: pgsql/doc/src/sgml/maintenance.sgml,v 1.51 2005/11/04 23:14:00 pete
     command. The first form, known as <quote>lazy vacuum</quote> or
     just <command>VACUUM</command>, marks expired data in tables and
     indexes for future reuse; it does <emphasis>not</emphasis> attempt
-    to reclaim the space used by this expired data
-    immediately. Therefore, the table file is not shortened, and any
-    unused space in the file is not returned to the operating
-    system. This variant of <command>VACUUM</command> can be run
-    concurrently with normal database operations.
+    to reclaim the space used by this expired data unless the space is
+    at the end of the table and an exclusive lock can be obtained easily
+    on the table. Unused space at the start or middle of the file does
+    not result in the file being shortened and space returned to the
+    operating system. This variant of <command>VACUUM</command> can be
+    run concurrently with normal database operations.
    </para>
 
    <para>