Issue 12090 - Make std.concurrency compatible with fibers as threads

https://d.puremagic.com/issues/show_bug.cgi?id=12090
Maybe replace the first part with "The Mutex to associate with the Condition" to clarify that the scheduler is responsible for making the first "will be" happen right now, whereas the caller is responsible for guaranteeing the second "will be" at some point in the future.