Remind reviewer when the FCP finishes

[RalfJung]

When an FCP finishes, the PR should go back into the reviewer's queue. However, for reviewers that manage their queue via github notifications or emails, this is far from clear -- the RFC is weeks, often months, old at this point, and they might have forgotten that they are assigned reviewer.

Would be good if the rfcbot message on FCP completion could state who the reviewer is to remind them that "ah, I should finally review this now".