Update 8.15.1.asciidoc (elastic#113221)

docs/reference/release-notes/8.15.1.asciidoc

@@ -23,6 +23,14 @@ To work around this issue, you have a number of options:
 <<esql-kibana-enable,disable ES|QL queries in {kib}>>
 ** Change the default data view in Discover to a smaller set of indices and/or one with fewer mapping conflicts.
 
+* Index Stats, Node Stats and Cluster Stats API can return a null pointer exception if an index contains a `dense_vector` field 
+but there is an index segment that does not contain any documents with a dense vector field ({es-pull}112720[#112720]). Workarounds:
+** If the affected index already contains documents with a dense vector field, force merge the index to a single segment.
+** If the affected index does not already contain documents with a dense vector field, index a document with a dense vector field
+and then force merge to a single segment.
+** If the affected index's `dense_vector` fields are unused, reindex without the `dense_vector` fields.
+
+
 [[bug-8.15.1]]
 [float]
 === Bug fixes