PropertyConverter: add note about not using BoxStore, handling nulls.

objectbox#563

Author: greenrobot-team

objectbox-java-api/src/main/java/io/objectbox/converter/PropertyConverter.java

@@ -23,6 +23,8 @@
  * <ul>
  * <li>Converters are created by the default constructor</li>
  * <li>Converters must be implemented thread-safe</li>
+ * <li>Converters must not interact with the database (such as using Box or BoxStore)</li>
+ * <li>Converters must handle null values</li>
  * </ul>
  */
 public interface PropertyConverter<P, D> {