Clarify what update="false" in <zip> does.

PR: 26443


git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@275994 13f79535-47bb-0310-9956-ffa450edef68

docs/manual/CoreTasks/zip.html

@@ -41,10 +41,11 @@ or by using the special nested fileset <i>zipgroupfileset</i>.</p>
 file already exists. When set to <code>yes</code>, the ZIP file is
 updated with the files specified. (New files are added; old files are
 replaced with the new versions.) When set to <code>no</code> (the
-default) the ZIP file is overwritten.  Please note that ZIP files
-store file modification times with a granularity of two seconds.  If a
-file is less than two seconds newer than the entry in the archive, Ant
-will not consider it newer.</p>
+default) the ZIP file is overwritten if any of the files that would be
+added to the archive are newer than the entries inside the archive.
+Please note that ZIP files store file modification times with a
+granularity of two seconds.  If a file is less than two seconds newer
+than the entry in the archive, Ant will not consider it newer.</p>
 
 <p>The <code>whenempty</code> parameter controls what happens when no files match.
 If <code>skip</code> (the default), the ZIP is not created and a warning is issued.
@@ -240,7 +241,7 @@ The code
 <p>
 <p>zips all files in the <code>htdocs/manual</code> directory into the <code>docs/user-guide</code> directory in the archive and includes all the files in any file that maches <code>examples*.zip</code>, such as all files within <code>examples1.zip</code> or <code>examples_for_brian.zip</code>.
 <hr>
-<p align="center">Copyright &copy; 2000-2003 Apache Software Foundation. All rights
+<p align="center">Copyright &copy; 2000-2004 Apache Software Foundation. All rights
 Reserved.</p>
 
 </body>