The new builtin `anext` does not have a signature (from `inspect.signature(anext)`).  This is expected, because `inspect` does not yet support signatures with C NULL default value.  However, `anext` also doesn't have text in its docstring that describes its signature as is typical for builtins.  This means `help(anext)` in the Python REPL gives no indication how to call the function.  It should.

This is clearly an oversight.  See comment here: https://github.com/python/cpython/pull/23847#commitcomment-45318696

Also, the "default" argument is not described in the docstring.

Related issue: https://bugs.python.org/issue31861

(PR forthcoming)