Add information about namespace encoding to API docs
What does this MR do?
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25722
Replaces Contribution MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8307
Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/25722
Replaces Contribution MR https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/8307