Add note on expose_path helper usage to API docs

doc/development/api_styleguide.md

@@ -92,5 +92,18 @@ For instance:
 Model.create(foo: params[:foo])
 ```
 
+## Using API path helpers in GitLab Rails codebase
+
+Because we support [installing GitLab under a relative URL], one must take this
+into account when using API path helpers generated by Grape. Any such API path
+helper usage must be in wrapped into the `expose_path` helper call.
+
+For instance:
+
+```haml
+- endpoint = expose_path(api_v4_projects_issues_related_merge_requests_path(id: @project.id, issue_iid: @issue.iid))
+```
+
 [Entity]: https://gitlab.com/gitlab-org/gitlab-ce/blob/master/lib/api/entities.rb
 [validation, and coercion of the parameters]: https://github.com/ruby-grape/grape#parameter-validation-and-coercion
+[installing GitLab under a relative URL]: https://docs.gitlab.com/ee/install/relative_url.html