IO-531 - Improve Javadoc for FileUtils.listFiles

This fixes #33

Author: sebbASF

src/main/java/org/apache/commons/io/FileUtils.java

@@ -503,10 +503,11 @@ private static void innerListFiles(final Collection<File> files, final File dire
      * in <code>FileFilterUtils.makeCVSAware(null)</code>.
      *
      * @param directory  the directory to search in
-     * @param fileFilter filter to apply when finding files.
+     * @param fileFilter filter to apply when finding files. Must not be {@code null},
+     *                   use {@link TrueFileFilter.INSTANCE} to match all files in selected directories. 
      * @param dirFilter  optional filter to apply when finding subdirectories.
      *                   If this parameter is {@code null}, subdirectories will not be included in the
-     *                   search. Use TrueFileFilter.INSTANCE to match all directories.
+     *                   search. Use {@link TrueFileFilter.INSTANCE} to match all directories.
      * @return a collection of java.io.File with the matching files
      * @see org.apache.commons.io.filefilter.FileFilterUtils
      * @see org.apache.commons.io.filefilter.NameFileFilter