[DOCS] Add ML breaking change for 6.6 (#37582)

Author: lcawl

docs/reference/migration/migrate_6_6.asciidoc

@@ -7,7 +7,10 @@
 This section discusses the changes that you need to be aware of when migrating
 your application to Elasticsearch 6.6.
 
+* <<breaking_66_ml_changes>>
+* <<breaking_66_mapping_changes>>
 * <<breaking_66_search_changes>>
+* <<breaking_66_setting_changes>>
 
 See also <<release-highlights>> and <<es-release-notes>>.
 
@@ -81,6 +84,7 @@ GET /stackoverflow/_search
 // TEST[setup:stackoverflow]
 
 [float]
+[[breaking_66_setting_changes]]
 === Settings changes
 
 [float]
@@ -108,6 +112,7 @@ the cluster settings API.
 `xpack.notification.slack.account.<id>.secure_url`
 
 [float]
+[[breaking_66_mapping_changes]]
 === Mappings changes
 
 [float]
@@ -136,4 +141,13 @@ previously created indexes.
 
 The following type parameters are deprecated for the `geo_shape` field type: `tree`,
 `precision`, `tree_levels`, `distance_error_pct`, `points_only`, and `strategy`. They
-will be removed in a future version.
+will be removed in a future version.
+
+[float]
+[[breaking_66_ml_changes]]
+=== Machine learning changes
+
+The get jobs API and get job stats API can retrieve a maximum of 10,000 jobs.
+Likewise, the get datafeeds API and get datafeed stats API can retrieve a
+maximum of 10,000 datafeeds. Prior to version 6.6, there were no limits on the
+results from these APIs.