bpo-43918: document signature and default argument of anext builtin

Misc/NEWS.d/next/Core and Builtins/2021-04-23-03-46-45.bpo-43918.nNDY3S.rst

@@ -0,0 +1 @@
+Document the signature and ``default`` argument in the docstring of the new ``anext`` builtin.

Python/bltinmodule.c

@@ -1635,13 +1635,16 @@ anext as builtin_anext
     default: object = NULL
     /
 
-Return the next item from the async iterator.
+async anext(aiterator[, default])
+
+Return the next item from the async iterator.  If default is given and the async
+iterator is exhausted, it is returned instead of raising StopAsyncIteration.
 [clinic start generated code]*/
 
 static PyObject *
 builtin_anext_impl(PyObject *module, PyObject *aiterator,
                    PyObject *default_value)
-/*[clinic end generated code: output=f02c060c163a81fa input=699d11f4e38eca24]*/
+/*[clinic end generated code: output=f02c060c163a81fa input=8f63f4f78590bb4c]*/
 {
     PyTypeObject *t;
     PyObject *awaitable;