Add note and link to 'tune for disk usage' (#23252)

* Add note and link to 'tune for disk usage'

* Changed formatting as suggested

Thanks, @ clintongormley!

docs/reference/how-to/indexing-speed.asciidoc

@@ -113,3 +113,9 @@ indexing.
 The default is `10%` which is often plenty: for example, if you give the JVM
 10GB of memory, it will give 1GB to the index buffer, which is enough to host
 two shards that are heavily indexing.
+
+[float]
+=== Additional optimizations
+
+Many of the strategies outlined in <<tune-for-disk-usage>> also
+provide an improvement in the speed of indexing.