bpo-43918: document signature and default argument of anext builtin (#25551)

Co-authored-by: Dennis Sweeney <36520290+sweeneyde@users.noreply.github.com>

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

@@ -1631,13 +1631,16 @@ 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;

Python/clinic/bltinmodule.c.h

@@ -543,7 +543,10 @@ PyDoc_STRVAR(builtin_anext__doc__,
 "anext($module, aiterator, default=<unrepresentable>, /)\n"
 "--\n"
 "\n"
-"Return the next item from the async iterator.");
+"async anext(aiterator[, default])\n"
+"\n"
+"Return the next item from the async iterator.  If default is given and the async\n"
+"iterator is exhausted, it is returned instead of raising StopAsyncIteration.");
 
 #define BUILTIN_ANEXT_METHODDEF    \
     {"anext", (PyCFunction)(void(*)(void))builtin_anext, METH_FASTCALL, builtin_anext__doc__},
@@ -874,4 +877,4 @@ builtin_issubclass(PyObject *module, PyObject *const *args, Py_ssize_t nargs)
 exit:
     return return_value;
 }
-/*[clinic end generated code: output=da9ae459e9233259 input=a9049054013a1b77]*/
+/*[clinic end generated code: output=e1d8057298b5de61 input=a9049054013a1b77]*/