api-docs: Make small clean-ups to feature level 254 updates.

Updates the changelog entry to use "ID" instead of "id".

Revises text for "null" value to match other field descriptions
in these stream/subscription objects.

api_docs/changelog.md

@@ -26,8 +26,8 @@ format used by the Zulip server that they are interacting with.
   [`GET /streams`](/api/get-streams),
   [`GET /streams/{stream_id}`](/api/get-stream-by-id),
   [`GET /users/me/subscriptions`](/api/get-subscriptions): Added a new
-  field `creator_id`, on stream and subscription objects, containing the
-  user id of the stream creator.
+  field `creator_id` to stream and subscription objects, which contains the
+  user ID of the stream's creator.
 
 **Feature level 253**
 

zerver/openapi/zulip.yaml

@@ -19235,9 +19235,9 @@ components:
           description: |
             The ID of the user who created this stream.
 
-            Is `null` for streams with no recorded creator, often because they are very
-            old, or were created via a data import tool or [management
-            command][management-commands].
+            A `null` value means the stream has no recorded creator, which is often
+            because the stream is very old, or because it was created via a data
+            import tool or [management command][management-commands].
 
             **Changes**: New in Zulip 9.0 (feature level 254).
 
@@ -19974,9 +19974,9 @@ components:
           description: |
             The ID of the user who created this stream.
 
-            Is `null` for streams with no recorded creator, often because they are very
-            old, or were created via a data import tool or [management
-            command][management-commands].
+            A `null` value means the stream has no recorded creator, which is often
+            because the stream is very old, or because it was created via a data
+            import tool or [management command][management-commands].
 
             **Changes**: New in Zulip 9.0 (feature level 254).