add note for reason for using setproperty rather than setnewproperty in available.java

git-svn-id: https://svn.apache.org/repos/asf/ant/core/trunk@277338 13f79535-47bb-0310-9956-ffa450edef68

src/main/org/apache/tools/ant/taskdefs/Available.java

@@ -213,6 +213,8 @@ public class Available extends Task implements Condition {
                         + "  Build file should not reuse the same property"
                         + " name for different values.");
                 }
+                // NB: this makes use of Project#setProperty rather than Project#setNewProperty
+                //     due to backwards compatiblity reasons
                 getProject().setProperty(property, value);
             }
         } finally {