javadoc: null val means no attr

git-svn-id: https://svn.apache.org/repos/asf/lucene/solr/trunk@883240 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/solr/request/XMLWriter.java

@@ -186,6 +186,7 @@ final public class XMLWriter {
     defaultIndent = doIndent;
   }
 
+  /** Writes the XML attribute name/val. A null val means that the attribute is missing. */
   public void writeAttr(String name, String val) throws IOException {
     if (val != null) {
       writer.write(' ');