improve javadoc

Author: vladak

opengrok-indexer/src/main/java/org/opengrok/indexer/history/CacheException.java

@@ -26,6 +26,7 @@
 
 /**
  * Exception thrown when retrieval or manipulation of cache data fails.
+ * This is used for implementations of {@link HistoryCache} and {@link AnnotationCache}.
  */
 public class CacheException extends Exception {
     private static final long serialVersionUID = 1L;