[ML][Data Frame] adding breaking changes to doc (#45051) (#45085)

* [ML][Data Frame] adding breaking changes to doc

* [DOCS] Edits data frame transform breaking change

* Update migrate_7_3.asciidoc

Author: benwtrent

docs/reference/migration/migrate_7_3.asciidoc

@@ -14,6 +14,16 @@ See also <<release-highlights>> and <<es-release-notes>>.
 
 //tag::notable-breaking-changes[]
 [discrete]
+[[breaking_73_api_changes]]
+=== API changes
+
+[discrete]
+==== {dataframe-transform-cap} API changes
+
+It is no longer possible to supply the `format` parameter when you define a
+`date_histogram` {dataframe-transform} pivot. Previously constructed transforms 
+will still run but the configured `format` will be ignored.
+
 [[breaking_73_mapping_changes]]
 === Mapping changes
 `dense_vector` field now requires `dims` parameter, specifying the number of