Document subscription API endpoints

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It would be great to extend our API documentation (doc/api/namespaces.md) with subscription-related endpoints (ee/lib/ee/api/namespaces.rb) because these seem to be not documented now.

The following discussion from !20079 (merged) should be addressed:

• @jprovaznik started a discussion: (+1 comment)

  no change required, todo for myself: Normally I would ask also for updating API doc, but it seems we have missing/incomplete API documentation for namespaces 😞 - I think we should address this separately - I'll create an issue for this.