Improve changelog entry for JENKINS-21322

changelog.html

@@ -67,7 +67,7 @@ Upcoming changes</a>
     NPE when /script used on offline slave.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-26751">issue 26751</a>)
   <li class=rfe>
-    Cleanup workspace when last build is >X days ago.
+    Make periodic workspace cleanup configurable through system properties.
     (<a href="https://issues.jenkins-ci.org/browse/JENKINS-21322">issue 21322</a>)
   <li class=bug>
     Do not offer to restart on /restart and /safeRestart if the configuration does not support it.