bpo-45250: fix docs regarding __iter__ and iterators being inconsistently required by CPython (GH-29170)

It is now considered a historical accident that e.g. `for` loops and the `iter()` built-in function do not require the iterators they work with to define `__iter__`, only `__next__`.

Doc/c-api/iter.rst

@@ -9,8 +9,8 @@ There are two functions specifically for working with iterators.
 
 .. c:function:: int PyIter_Check(PyObject *o)
 
-   Return non-zero if the object *o* supports the iterator protocol, and ``0``
-   otherwise.  This function always succeeds.
+   Return non-zero if the object *o* can be safely passed to
+   :c:func:`PyIter_Next`, and ``0`` otherwise.  This function always succeeds.
 
 .. c:function:: int PyAIter_Check(PyObject *o)
 
@@ -21,10 +21,11 @@ There are two functions specifically for working with iterators.
 
 .. c:function:: PyObject* PyIter_Next(PyObject *o)
 
-   Return the next value from the iteration *o*.  The object must be an iterator
-   (it is up to the caller to check this).  If there are no remaining values,
-   returns ``NULL`` with no exception set.  If an error occurs while retrieving
-   the item, returns ``NULL`` and passes along the exception.
+   Return the next value from the iterator *o*.  The object must be an iterator
+   according to :c:func:`PyIter_Check` (it is up to the caller to check this).
+   If there are no remaining values, returns ``NULL`` with no exception set.
+   If an error occurs while retrieving the item, returns ``NULL`` and passes
+   along the exception.
 
 To write a loop which iterates over an iterator, the C code should look
 something like this::

Doc/c-api/typeobj.rst

@@ -1521,9 +1521,9 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
 .. c:member:: getiterfunc PyTypeObject.tp_iter
 
-   An optional pointer to a function that returns an iterator for the object.  Its
-   presence normally signals that the instances of this type are iterable (although
-   sequences may be iterable without this function).
+   An optional pointer to a function that returns an :term:`iterator` for the
+   object.  Its presence normally signals that the instances of this type are
+   :term:`iterable` (although sequences may be iterable without this function).
 
    This function has the same signature as :c:func:`PyObject_GetIter`::
 
@@ -1536,8 +1536,8 @@ and :c:type:`PyType_Type` effectively act as defaults.)
 
 .. c:member:: iternextfunc PyTypeObject.tp_iternext
 
-   An optional pointer to a function that returns the next item in an iterator.
-   The signature is::
+   An optional pointer to a function that returns the next item in an
+   :term:`iterator`. The signature is::
 
       PyObject *tp_iternext(PyObject *self);
 
@@ -2429,8 +2429,8 @@ Async Object Structures
 
       PyObject *am_await(PyObject *self);
 
-   The returned object must be an iterator, i.e. :c:func:`PyIter_Check` must
-   return ``1`` for it.
+   The returned object must be an :term:`iterator`, i.e. :c:func:`PyIter_Check`
+   must return ``1`` for it.
 
    This slot may be set to ``NULL`` if an object is not an :term:`awaitable`.