OPENNLP-1886: Document the StemmerUtil constructor

Author: krickert

opennlp-core/opennlp-runtime/src/main/java/opennlp/tools/stemmer/light/StemmerUtil.java

@@ -23,6 +23,7 @@
  */
 final class StemmerUtil {
 
+  /** Prevents construction of this utility class. */
   private StemmerUtil() {
   }