Add changelog note for the directio option

Author: thecoop

docs/changelog/125921.yaml

@@ -0,0 +1,18 @@
+pr: 125921
+summary: Allow direct IO for BBQ rescoring
+area: Vector Search
+type: feature
+highlight:
+  title: Allow direct IO for BBQ rescoring
+  body: |-
+    BBQ rescoring can be drastically affected by the amount of available RAM
+    for use in the page cache. When there is not enough RAM to fit the whole
+    vector data in memory, BBQ search latencies can be affected by as much as 5000x.
+    Specifying the `vector.rescoring.directio=true` Java option on all vector search
+    nodes changes rescoring to use direct IO, which eliminates these very high latencies
+    from searches in low-memory scenarios, at a cost of a reduction
+    in vector search performance for BBQ indices.
+
+    This option is released in 9.1 as a tech preview whilst we analyse its effect
+    for a variety of use cases.
+issues: []