Update stemmer-override-tokenfilter.asciidoc
percolator: fix handling of nested documents

Nested documents were indexed as separate documents, but it was never checked
if the hits represent nested documents or not. Therefore, nested objects could
match not nested queries and nested queries could also match not nested documents.

Examples are in issue #6540 .

closes #6540
closes #6544
Aggregations: Added percentile rank aggregation

Percentile Rank Aggregation is the reverse of the Percetiles aggregation.  It determines the percentile rank (the proportion of values less than a given value) of the provided array of values.

Closes #6386