Add Percentiles breaking changed documentation

Author: flobernd

docs/release-notes/breaking-changes.md

@@ -19,6 +19,26 @@ Breaking changes can impact your Elastic applications, potentially disrupting no
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+## 9.1.1 [elasticsearch-net-client-911-breaking-changes]
+
+### Overview
+
+- [1. Improved usability of `Percentiles` aggregation results](#1-percentiles-aggregate)
+
+### Breaking changes
+
+#### 1. Improved usability of `Percentiles` aggregation results [#1-percentiles-aggregate]
+
+**Impact**: Low.
+
+The type of the `Values` property for the following classes got changed from the `Percentiles` union to a simple `PercentilesItem` collection:
+
+- `HdrPercentilesAggregate`
+- `PercentilesBucketAggregate`
+- `TDigestPercentilesAggregate`
+
+The `Percentiles` union type got removed as it's no longer used.
+
 ## 9.0.0 [elasticsearch-net-client-900-breaking-changes]
 
 ### Overview