Add visibility change to API v3 -> v4 doc

And add changelog entry.

changelogs/unreleased/27501-api-use-visibility-everywhere.yml

+---
+title: "API: Use `visibility` as string parameter everywhere"
+merge_request: 9337
+author:

doc/api/v3_to_v4.md

@@ -47,6 +47,7 @@ changes are in V4:
   - PUT `projects/:id`
 - Renamed `branch_name` to `branch` on DELETE `id/repository/branches/:branch` response [!8936](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8936)
 - Remove `public` param from create and edit actions of projects [!8736](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8736)
+- Use `visibility` as string parameter everywhere [!9337](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9337)
 - Notes do not return deprecated field `upvote` and `downvote` [!9384](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9384)
 - Return HTTP status code `400` for all validation errors when creating or updating a member instead of sometimes `422` error. [!9523](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9523)
 - Remove `GET /groups/owned`. Use `GET /groups?owned=true` instead [!9505](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/9505)