[DOCS] Thread pool settings are static (#81887)

Starting in 5.1 Thread Pools can no longer be dynamically updated, [doc](https://www.elastic.co/guide/en/elasticsearch/reference/5.0/breaking_50_settings_changes.html#_threadpool_settings).

docs/reference/modules/threadpool.asciidoc

@@ -112,9 +112,10 @@ There are several thread pools, but the important ones include:
     `min(5 * (`<<node.processors, `# of allocated processors`>>`), 50)`
     and queue_size of `1000`.
 
-Changing a specific thread pool can be done by setting its type-specific
-parameters; for example, changing the number of threads in the `write` thread
-pool:
+Thread pool settings are <<static-cluster-setting,static>> and can be changed by
+editing `elasticsearch.yml`. Changing a specific thread pool can be done by
+setting its type-specific parameters; for example, changing the number of
+threads in the `write` thread pool:
 
 [source,yaml]
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