Add changelog requirements for REST/GraphQL API docs

doc/development/documentation/graphql_styleguide.md

@@ -90,3 +90,7 @@ Be aware that CI tests for that second MR will fail with a bad link until the
 main MR that adds the new GraphQL page is merged. Therefore, only merge the MR against the
 [`gitlab-docs`](https://gitlab.com/gitlab-org/gitlab-docs) repository after the content has
 been merged and live on `docs.gitlab.com`.
+
+## Include a changelog entry
+
+All client-facing changes **must** include a [changelog entry](../changelog.md).

doc/development/documentation/restful_api_styleguide.md

@@ -178,3 +178,8 @@ do something like this:
 ```shell
 curl --request PUT --header "PRIVATE-TOKEN: <your_access_token>" --data "skip_users[]=<user_id>" --data "skip_users[]=<user_id>" "https://gitlab.example.com/api/v4/projects/<project_id>/users"
 ```
+
+## Include a changelog entry
+
+All client-facing changes **must** include a [changelog entry](../changelog.md).
+This does not include internal APIs.