add thread pool change availability (elastic#131734)

Author: shainaraskas

docs/reference/elasticsearch/configuration-reference/thread-pool-settings.md

@@ -35,7 +35,12 @@ $$$search-throttled$$$`search_throttled`
 `write`
 :   For write operations and ingest processors. Thread pool type is `fixed` with a size of [`# of allocated processors`](#node.processors), queue_size of `max(10000, (`[`# of allocated processors`](#node.processors)`* 750))`. The maximum size for this pool is `1 + `[`# of allocated processors`](#node.processors).
 
-`write_coordination`
+    :::{note}
+    In {{stack}} 9.0 and earlier, the `write` thread pool was also used for bulk requests.
+    In {{stack}} 9.1 and earlier, the queue_size was 10000.
+    :::
+
+`write_coordination` {applies_to}`stack: ga 9.1`
 :   For bulk request coordination operations. Thread pool type is `fixed` with a size of [`# of allocated processors`](#node.processors), queue_size of `10000`. The maximum size for this pool is `1 + `[`# of allocated processors`](#node.processors).
 
 `snapshot`
@@ -74,7 +79,7 @@ $$$search-throttled$$$`search_throttled`
 `system_write`
 :   For write operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
 
-`system_write_coordination`
+`system_write_coordination` {applies_to}`stack: ga 9.1`
 :   For bulk request coordination operations on system indices. Thread pool type is `fixed` with a default maximum size of `min(5, (`[`# of allocated processors`](#node.processors)`) / 2)`.
 
 `system_critical_read`