Add xlm roberta to the list of supported NLP model architectures (#2478)

Author: davidkyle

docs/en/stack/ml/nlp/ml-nlp-model-ref.asciidoc

@@ -33,6 +33,7 @@ The current list of supported architectures is:
 * RetriBERT
 * MPNet
 * SentenceTransformers bi-encoders with the above transformer architectures
+* XLM-RoBERTa
 
 In general, any trained model that has a supported architecture is deployable in
 {es} by using eland. However, it is not possible to test every third party