PositionMsg: Added note on invalid position

Author: fixje

src/main/java/de/serosystems/lib1090/msgs/PositionMsg.java

@@ -11,7 +11,8 @@ public interface PositionMsg {
     boolean hasValidPosition();
 
     /**
-     * @return the CPR encoded position that was announced in this message
+     * @return the CPR encoded position that was announced in this message.
+     * The method may return an invalid position if #hasValidPosition() is false.
      */
     CPREncodedPosition getCPREncodedPosition();