Make joinWithNever attempting self cancelling

[lenguyenthanh]

Close #4093 by attempting self-cancel before F.never

[djspiewak]

So the concern I have with this is that there's a lot of call-sites which kind of implicitly assume it isn't self-canceling. This is a pretty significant semantic to change. The inconsistency with the docs was actually reported back before 3.0.0 went final, and we decided that the mistaken semantic (the one presently in HEAD) was more expected, but we never went back and fixed the docs.

It think it might be better to fix the docs to match the semantic, rather than changing something like this which can have really far-reaching consequences.

[armanbilge]

See also a similar confusion with embedNever in #3351 (comment).

[lenguyenthanh]

that makes sense. I'll submit another pr for updating the docs.