adjust the existing message to better explain what it does

Author: Vladimir Kotal

opengrok-indexer/src/main/java/org/opengrok/indexer/index/Indexer.java

@@ -566,8 +566,9 @@ public static String[] parseOptions(String[] argv) throws ParseException {
             parser.on("-H", "--history", "Enable history.").execute(v -> cfg.setHistoryEnabled(true));
 
             parser.on("--historyThreads", "=number", Integer.class,
-                    "The number of threads to use for history cache generation. By default the number",
-                    "of threads will be set to the number of available CPUs. Assumes -H/--history.").execute(threadCount ->
+                    "The number of threads to use for history cache generation on repository level. " +
+                    "By default the number of threads will be set to the number of available CPUs.",
+                    "Assumes -H/--history.").execute(threadCount ->
                     cfg.setHistoryParallelism((Integer) threadCount));
 
             parser.on("--historyFileThreads", "=number", Integer.class,