I'm following the "status-completed" tag on MetaSO to see what changes are going in. Sometimes I see questions which request something which already exists (like this one, for example), and they get tagged "status-completed". This is a little misleading, since you didn't actually implement/change/complete anything. Maybe another status-* badge can be used? Either "status-bydesign" or a new one (I can't think of a good name for it though.. maybe "status-alreadythere"?).
Status-Completed represents the fact that this 'feature request' has been implemented and it doesn't matter when it was implemented. The fact remains that what the person has requested is already available to them. They should be delighted!
Status-bydesign means that what has been reported as a bug was in fact a particular choice by the designers and that they will not be changing that functionality.
Status-alreadythere would only be equivalent to
status-RTFM if the feature were not only coded but also documented.