Missing docs for updateIndexAsync on Cblite java platform

First comment:

From @Martin-Hogge in https://github.com/couchbase/couchbase-lite-ios/issues/1010#issuecomment-296972275

> Hi, I correctly see the updateIndex reference on the official doc but none for the updateIndexAsync. I could run the updateIndex method on a background thread myself but if an async method exists it would be easier to directly use it.

> Plus, I do not see it in the official documentation but I find out that their is an updateIndexAlone method on the CBLite sdk. I'm not sure to understand correctly the difference between updateIndex and updateIndexAlone. Here is its JavaDoc:

```
/**

• Updates the view's index (incrementally) if necessary.
• Only the view's index alone will be updated.
  *
• @return Status OK if updated or NOT_MODIFIED if already up-to-date.
• @throws CouchbaseLiteException
  */
  ```

see other comments: https://github.com/couchbaselabs/couchbase-mobile-portal/issues/681