api-docs: Adds a Changes note for `is_owner` in register response.

Adds a feature level 11 note to the `is_owner` field in the
`POST /register` response and revises the changelog entry
for the same.

api_docs/changelog.md

@@ -1550,8 +1550,9 @@ No changes; feature level used for Zulip 3.0 release.
   time limit before community topic editing is forbidden.  A `null`
   value means no limit. This was previously hard-coded in the server
   as 86400 seconds (1 day).
-* [`POST /register`](/api/register-queue): The response now contains a
-  `is_owner`, similar to the existing `is_admin` and `is_guest` fields.
+* [`POST /register`](/api/register-queue): The response now contains
+  an `is_owner` boolean field, which is similar to the existing
+  `is_admin` and `is_guest` fields.
 * [`POST /typing`](/api/set-typing-status): Removed legacy
   support for sending email addresses in the `to` parameter, rather
   than user IDs, to encode direct message recipients.

zerver/openapi/zulip.yaml

@@ -14487,6 +14487,8 @@ paths:
                           Present if `realm_user` is present in `fetch_event_types`.
 
                           Whether the current user is an [organization owner](/api/roles-and-permissions).
+
+                          **Changes**: New in Zulip 3.0 (feature level 11).
                       is_billing_admin:
                         type: boolean
                         description: |