add some documentation hints

see https://bz.apache.org/bugzilla/show_bug.cgi?id=62076

src/main/org/apache/tools/ant/util/ChainedMapper.java

@@ -35,6 +35,9 @@ public class ChainedMapper extends ContainerMapper {
     public String[] mapFileName(String sourceFileName) {
         List inputs = new ArrayList();
         List results = new ArrayList();
+        // we do this even if the sourceFileName is null. Some
+        // resources have a null name and users may use a mapper like
+        // MergeMapper to provide a name for it.
         results.add(sourceFileName);
         FileNameMapper mapper = null;
 

src/main/org/apache/tools/ant/util/FileNameMapper.java

@@ -52,7 +52,9 @@ public interface FileNameMapper {
      * omit the source file in question.</p>
      *
      * @param sourceFileName the name of the source file relative to
-     *                       some given basedirectory.
+     *                       some given basedirectory. Might be {@code
+     *                       null} for resources that don't provide a
+     *                       name.
      * @return an array of strings if the rule applies to the source file, or
      *         null if it does not.
      */