As described at https://bugs.python.org/issue43918, the new anext builtin doesn't include any signature information. help(anext) should show something like anext(aiterator[, default]), which is added in this PR.
I also added text describing the default argument as I understand it. This text was adapted from next. See the description in the documentation here: https://github.com/python/cpython/pull/23847/files#diff-6a7a07ac473fdd76734669b1b70626ad2176011129902f6add017810f54d0439R87
I ran the clinic on " Python/bltinmodule.c".
Alright, now I need to figure out how to add a blurb...