Add information about the removal of store throttling to the migration guide.

Relates to #21573

docs/reference/migration/migrate_6_0.asciidoc

@@ -25,6 +25,7 @@ way to reindex old indices is to use the `reindex` API.
 === Also see:
 
 * <<breaking_60_cat_changes>>
+* <<breaking_60_stats_changes>>
 * <<breaking_60_rest_changes>>
 * <<breaking_60_search_changes>>
 * <<breaking_60_docs_changes>>
@@ -35,6 +36,8 @@ way to reindex old indices is to use the `reindex` API.
 
 include::migrate_6_0/cat.asciidoc[]
 
+include::migrate_6_0/stats.asciidoc[]
+
 include::migrate_6_0/rest.asciidoc[]
 
 include::migrate_6_0/search.asciidoc[]

docs/reference/migration/migrate_6_0/settings.asciidoc

@@ -7,4 +7,10 @@ The internal setting `cluster.routing.allocation.snapshot.relocation_enabled` th
 different nodes has been removed. Enabling this setting could cause allocation issues if a shard got allocated off a node and then
 reallocated back to this node while a snapshot was running.
 
+==== Store throttling settings
 
+Store throttling has been removed. As a consequence, the
+`indices.store.throttle.type` and `indices.store.throttle.max_bytes_per_sec`
+cluster settings and the `index.store.throttle.type` and
+`index.store.throttle.max_bytes_per_sec` index settings are not
+recognized anymore.

docs/reference/migration/migrate_6_0/stats.asciidoc

@@ -0,0 +1,7 @@
+[[breaking_60_stats_changes]]
+=== Stats and info changes
+
+==== Removal of `throttle_time` in the `store` stats
+
+Given that store throttling has been removed, the `store` stats do not report
+`throttle_time` anymore.