Group index: Minor cleanups

* Document is:visibletoall in all lower case to be consistent with the
  case of other search operators (internally it's case-insensitive).
* Fix some comments.

Change-Id: Ied4c33a6968499b7f6dd9e74f3c25f48fb6bf401
Signed-off-by: Edwin Kempin <ekempin@google.com>

gerrit-server/src/main/java/com/google/gerrit/server/group/Index.java

@@ -52,7 +52,7 @@ public class Index implements RestModifyView<GroupResource, Input> {
           .format("External Group Not Allowed: %s", rsrc.getGroupUUID().get()));
     }
 
-    // evicting the group from the cache, reindexes the account
+    // evicting the group from the cache, reindexes the group
     groupCache.evict(group);
     return Response.none();
   }