update javadoc on DefaultPrettyPrinter (#1017)

Author: pjfanning

src/main/java/com/fasterxml/jackson/core/util/DefaultPrettyPrinter.java

@@ -12,6 +12,10 @@
  * method {@link JsonGenerator#useDefaultPrettyPrinter} is
  * used, which will use an instance of this class for operation.
  *
+ * This class is designed for the JSON data format. If you are using
+ * a different data format, you should use a {@link PrettyPrinter}
+ * implementation designed to support that format.
+ *
  * If you override this class, take note of {@link Instantiatable},
  * as subclasses will still create an instance of DefaultPrettyPrinter.
  */