Checkstyle: Add missing JavaDoc

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/lang/trunk@1654144 13f79535-47bb-0310-9956-ffa450edef68

src/main/java/org/apache/commons/lang3/text/StrLookup.java

@@ -53,6 +53,12 @@ public abstract class StrLookup<V> {
         return NONE_LOOKUP;
     }
 
+    /**
+     * Creates a copy of the given properties instance.
+     * 
+     * @param input the Properties instance to copy.
+     * @return a copy of {@code input}.
+     */
     private static Properties copyProperties(Properties input) {
         if (input == null) {
             return null;