Document how currently blueprints status are being set

[viniarck]

Currently, some blueprints are being left with status draft, where historically it used to change to accepted in the first lines of the markdown table. I believe we should update accordingly to accepted when blueprints are ready to land, but also also explains hat if a draft PR land it's OK to submit subsequent incremental PRs, after that, if a significant change is needed then we'd can advise to submit a new blueprint when applicable to obsolete or to evolve a previous one.