api-changelog: Add 2.1.0 entry for realm_default_external_accounts.

Adds an API changelog note to 2.1 for the addition of
realm_default_external_accounts to the `/register-queue` response.

Also adds a Changes note to the field in the endpoint's response
API documentation.

The original commit that added it to that endpoint's response was
commit d7ee2aced1.

api_docs/changelog.md

@@ -1479,6 +1479,8 @@ No changes; feature level used for Zulip 3.0 release.
 
 ## Changes in Zulip 2.1
 
+* [`POST /register`](/api/register-queue): Added
+  `realm_default_external_accounts` to endpoint response.
 * [`GET /messages`](/api/get-messages): Added support for
   [search/narrow options](/api/construct-narrow) that use stream/user
   IDs to specify a message's sender, its stream, and/or its recipient(s).

zerver/openapi/zulip.yaml

@@ -13220,8 +13220,10 @@ paths:
                           Present if `realm` is present in `fetch_event_types`.
 
                           Dictionary where each entry describes a default external
-                          account type that can be configured with Zulip's custom
-                          profile fields feature.
+                          account type that can be configured with Zulip's [custom
+                          profile fields feature](/help/custom-profile-fields).
+
+                          **Changes**: New in Zulip 2.1.0.
                         additionalProperties:
                           description: |
                             `{site_name}`: Dictionary containing the details of the