Clarify in documentation the specs status

During the design - implementation - deployment process the documentation evolve and it might not be possible to capture that. Nevertheless it is important to clarify at what stage is the documentation.

It might also be useful to update a blueprint spec to the actual behavior of the implementation (but this is out of scope of this task)

In the Deposit documentation there are specs that describe:

1. an existing implementation
2. an ongoing implementation
3. a future feature that is in the roadmap ahead without a clear date for implementation

---

Migrated from T2751 (view on Phabricator)