fix(docs): add a note about lunr-folding

Not to use it (even though I "maintain" it) because it is not good

docs/languages.md

@@ -142,6 +142,10 @@ console.log(JSON.stringify(index.search("règlement de démolition")));
 # [{"ref":"1314-2023-DEM","score":0.4072935059634513,"matchData":{"metadata":{"regl":{"titre":{}},"demolit":{"titre":{}}}}}]
 ```
 
+There is also `lunr-folding` for JavaScript, but its folding is not
+the same as `unidecode` and it may not be fully compatible with
+language support, so it is recommended to use the above method.
+
 ## Notes on language support
 
 - Using multiple languages means the terms will be stemmed once per language. This can yield unexpected results.