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__`.

1 files changed, 7 insertions, 6 deletions

diff --git a/Doc/c-api/iter.rst b/Doc/c-api/iter.rst
index f7106f4..3e388bb 100644
--- a/Doc/c-api/iter.rst
+++ b/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::