api: Document API change for Stream.date_created.

templates/zerver/api/changelog.md

@@ -8,6 +8,17 @@ server versions is to check the `zulip_feature_level` parameter in the
 `/register` and `/server_settings` responses to determine which of the
 below features are supported.
 
+## Changes in Zulip 4.0
+
+**Feature Level 30**
+
+* [`GET users/me/subscriptions`](/api/get-subscriptions), [`GET
+  /streams`](/api/get-streams): Added `date_created` to Stream
+  objects.
+
+Feature levels 28 and 29 are reserved for future use in 3.x bug fix
+releases.
+
 ## Changes in Zulip 3.1
 
 **Feature level 27**

zerver/openapi/zulip.yaml

@@ -4126,6 +4126,8 @@ components:
           type: integer
           description: |
             The UNIX timestamp for when the stream was created, in UTC seconds.
+
+            **Changes**: New in Zulip 4.0 (feature level 30).
         invite_only:
           type: boolean
           description: |
@@ -4231,6 +4233,8 @@ components:
           type: integer
           description: |
             The UNIX timestamp for when the stream was created, in UTC seconds.
+
+            **Changes**: New in Zulip 4.0 (feature level 30).
         invite_only:
           type: boolean
           description: |