[DOCS] Adds pre-cleaning recommendation to ELSER docs. (#2796) (#2798)

(cherry picked from commit 34a6c7b)

Co-authored-by: István Zoltán Szabó <istvan.szabo@elastic.co>

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

@@ -440,6 +440,17 @@ per document to ingest.
 To learn more about ELSER performance, refer to the <<elser-benchmarks>>.
 
 
+[discrete]
+[[pre-cleaning]]
+== Pre-cleaning input text
+
+The quality of the input text significantly affects the quality of the embeddings.
+To achieve the best results, it's recommended to clean the input text before generating embeddings.
+The exact preprocessing you may need to do heavily depends on your text.
+For example, if your text contains HTML tags, use the {ref}/htmlstrip-processor.html[HTML strip processor] in an ingest pipeline to remove unnecessary elements.
+Always review and clean your input text before ingestion to eliminate any irrelevant entities that might affect the results.
+
+
 [discrete]
 [[elser-adaptive-allocations]]
 == Adaptive allocations