api docs: Document history of rate limit format.

templates/zerver/api/changelog.md

@@ -268,6 +268,8 @@ field with an integer field `invite_to_realm_policy`.
 
 * [`POST /users`](/api/create-user): Restricted access to organization
   administrators with the `can_create_users` permission.
+* [Error handling](/api/rest-error-hanlding): The `code` property will
+  not be present in errors due to rate limits.
 
 **Feature level 35**
 

templates/zerver/api/rest-error-handling.md

@@ -47,4 +47,7 @@ When the Zulip server has configured multiple rate limits that apply
 to a given request, the values returned will be for the strictest
 limit.
 
+**Changes**: The `code` field in the response is new in Zulip 4.0
+(feature level 36).
+
 {generate_code_example|/rest-error-handling:post|fixture(403)}