Feature test macro: __cpp_lib_list_remove_return_type

Looks like I used D0646R1 instead of the one from the straw polls page (which was dumb, as I wrote the Annex C wording so should have noticed it was missing). I'll push a commit to the branch ...

Done.

Uh, @jwakely: The commits should use sentence case, not title case, even if the paper title erred in that regard. Not sure how to fix that without a force-push, though.

I forgot that rule and did a copy&paste from the paper title, sorry.

@tkoeppe Right, but will @zygoloid remember this particular case?

Yes, @zygoloid reads these threads while rebasing and merging :)

Minor adjustments made while merging: changed restrictive "which" to "that" in Annex C, replaced \_ with _ (we use the textunderscore package), replaced "C++" with "\Cpp{}" macro in Annex C.