Docs: Mention we use DEFLATE for best_compression

Closes #13549

docs/reference/index-modules.asciidoc

@@ -71,7 +71,7 @@ Checking shards may take a lot of time on large indices.
 [[index-codec]] `index.codec`::
 
     experimental[] The `default` value compresses stored data with LZ4
-    compression, but this can be set to `best_compression` for a higher
+    compression, but this can be set to `best_compression` which uses https://en.wikipedia.org/wiki/DEFLATE[DEFLATE] for a higher
     compression ratio, at the expense of slower stored fields performance.
 
 [float]