add note about backslashes

git-svn-id: https://hudson.dev.java.net/svn/hudson/trunk/hudson/main@33346 71c3de6d-444a-0410-be80-ed276b4c234a

war/resources/help/ant/ant-properties.html

@@ -4,5 +4,6 @@
 name1=value1
 name2=value2
 </pre>
-  These are passed to Ant like <tt>"-Dname1=value1 -Dname2=value2"</tt>
+  These are passed to Ant like <tt>"-Dname1=value1 -Dname2=value2"</tt>.
+  Backslashes are used for escaping, so use "\\" for a single backslash.
 </div>