[8.8][DOCS] Adds ML-related release highlights to the what's new section. (#96175)

Author: szabosteve

docs/reference/release-notes/highlights.asciidoc

@@ -123,6 +123,18 @@ This PR removes the beta label for JWT realm feature to make it GA.
 
 {es-pull}95398[#95398]
 
+[discrete]
+[[elser-model]]
+=== The Elastic Learned Sparse EncodeR (ELSER) model
+
+In 8.8, we introduce the Elastic Learned Sparse EncodeR model to our {ml} model 
+library that you can use out of the box. ELSER improves the relevance of your 
+search results by enabling semantic search. This search method considers the 
+meaning of words rather than solely relying on literal terms. ELSER is a 
+pre-trained, out-of-domain sparse vector model that eliminates the need for 
+fine-tuning on your specific source data. It provides you with relevant search 
+results right from the start.
+
 // end::notable-highlights[]