[DOCS] Adds more conceptual info to ELSER (#2433)

Author: szabosteve

docs/en/stack/ml/nlp/ml-nlp-elser.asciidoc

@@ -20,9 +20,13 @@ meaning and user intent, rather than exact keyword matches.
 ELSER is an out-of-domain model which means it does not require fine-tuning on 
 your own data, making it adaptable for various use cases out of the box.
 
-Refer to 
-https://www.elastic.co/blog/may-2023-launch-information-retrieval-elasticsearch-ai-model[this blog post], 
-to learn more about ELSER.
+ELSER expands the indexed and searched passages into collections of terms that 
+are learned to co-occur frequently within a diverse set of training data. The 
+terms that the text is expanded into by the model _are not_ synonyms for the 
+search terms; they are learned associations. These expanded terms are weighted 
+as some of them are more significant than others. Then the {es} 
+{ref}/rank-feature.html[rank-feature field type] is used to store the terms and 
+weights at index time, and to search against later. 
 
 
 [discrete]