change gc_paginated_indexes default to true

addressed by #881
Almost all users don't look at here, so how about adding the entry
about `gc_paginated_indexes`?

It may be similar to `fold_objects_for_list_keys`:

```
              %% Set this to false if you're running
              %% Riak nodes prior to version 1.4.0.
              {fold_objects_for_list_keys, true},
```