Clarify when ${ant.project.invoked-targets} is set.

manual/properties.html

@@ -77,6 +77,9 @@ ant.project.invoked-targets
                     been specified on the command line (the IDE,
                     an <ant> task ...) when invoking the current
                     project.
+                    This property is set when the first target is executed.
+                    So you can't use it in the implicit target (directly
+                    under the <project> tag).
 ant.java.version    the JVM version Ant detected; currently it can hold
                     the values "1.7", "1.6", "1.5",
                     "1.4",  "1.3" and "1.2".