[DOC] knn.advanced.filtered_exact_search_threshold is mentioned as a cluster settings but it's not

[Rmaan]

This page lists knn.advanced.filtered_exact_search_threshold as a cluster setting. But if you try to apply it, you will get an error:

persistent setting [knn.advanced.filtered_exact_search_threshold], not recognized

After a some trial and error I dug into code and found it's actually an index level settings. The table need to be updated into 2 separated parts, or at least prefix index ones with index..

[hdhalter]

Thanks, @Rmaan ! @kolchfa-aws , can you please look into this? Thanks.