[m-rev.] for post-commit review: document requirement for non-abstract instances in interfaces

[Zoltan Somogyi]

2022-02-03 11:06 GMT+11:00 "Zoltan Somogyi" <zoltan.somogyi at runbox.com>:
> For review by Julien.

I forgot to mention: this one is for the release branch as well.

Zoltan.