Percentile aggregations are non-deterministic. A percentile aggregation
can produce different results even when using the same data.
Based on [this discuss post][0], the non-deterministic property stems
from processes in Lucene that can affect the order in which docs are
provided to the aggregation.
This adds a warning stating that the aggregation is non-deterministic
and what that means.
[0]: https://discuss.elastic.co/t/different-results-for-same-query/111757
694b119f0a