github-44: reference BuiltinFormats in javadocs for CellStyle.getDataFormat and setDataFormat

This closes #44 github pull request from sixinli

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1777260 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/ss/usermodel/CellStyle.java

@@ -338,14 +338,14 @@ public interface CellStyle {
     short getIndex();
 
     /**
-     * set the data format (must be a valid format)
+     * set the data format (must be a valid format). Built in formats are defined at {@link BuiltinFormats}.
      * @see DataFormat
      */
 
     void setDataFormat(short fmt);
 
     /**
-     * get the index of the format
+     * get the index of the data format. Built in formats are defined at {@link BuiltinFormats}.
      * @see DataFormat
      */
     short getDataFormat();