api_docs: Add missing followed topic visibility policy.

While the server implementation has accepted this value for a few
months as part of building the feature, following topics was not a
fully supported feature of the Zulip server before
3f2ab44f94, just before feature
level 219.

So that's probably the correct level to document as the first feature
level at which we recommend that clients supporting the followed
topics feature process the value.

api_docs/changelog.md

@@ -96,6 +96,10 @@ format used by the Zulip server that they are interacting with.
   [`PATCH /settings`](/api/update-settings): Renamed `default_view` and
   `escape_navigates_to_default_view` settings to `web_home_view` and
   `web_escape_navigates_to_home_view` respectively.
+* [`POST /user_topics`](/api/update-user-topic), [`POST
+  register`](/api/register-queue), [`GET /events`](/api/get-events):
+  Added followed as a supported value for visibility policies in
+  `user_topic` objects.
 
 **Feature level 218**