Add some information about the impact of index.codec setting. (elastic#110413) (elastic#110461)

Author: martijnvg

docs/reference/index-modules.asciidoc

@@ -79,7 +79,9 @@ breaking change].
     compression ratio, at the expense of slower stored fields performance.
     If you are updating the compression type, the new one will be applied
     after segments are merged. Segment merging can be forced using
-    <<indices-forcemerge,force merge>>.
+    <<indices-forcemerge,force merge>>. Experiments with indexing log datasets
+    have shown that `best_compression` gives up to ~18% lower storage usage
+    compared to `default` while only minimally affecting indexing throughput (~2%).
 
 [[routing-partition-size]] `index.routing_partition_size`::