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

https://d.puremagic.com/issues/show_bug.cgi?id=12090
As a user reading this, I'd wonder what a logical thread is, and how it relates to the term "asynchronous context" used elsewhere in the documentation. Of course, after looking at the implementation (or just all of the module, probably) it becomes clear, but maybe we can improve the documentation to make this clearer. The module-level docs might be a good place to do this, and should be updated to reflect the additions regardless (not just thread-level scheduling any longer, …).