gh-160669: Clarify PyDict_Update docs

Doc/c-api/dict.rst

@@ -472,11 +472,13 @@ Dictionary objects
 
 .. c:function:: int PyDict_Update(PyObject *a, PyObject *b)
 
-   This is the same as ``PyDict_Merge(a, b, 1)`` in C, and is similar to
-   ``a.update(b)`` in Python except that :c:func:`PyDict_Update` doesn't fall
-   back to the iterating over a sequence of key value pairs if the second
-   argument has no "keys" attribute.  Return ``0`` on success or ``-1`` if an
-   exception was raised.
+   This is the same as ``PyDict_Merge(a, b, 1)`` in C.
+
+   Similar to ``a.update(b)`` in Python except that :c:func:`PyDict_Update`
+   doesn't not support a sequence of key value pairs as *b* parameter.
+   Use :c:func:`PyDict_MergeFromSeq2` in this case.
+
+   Return ``0`` on success or ``-1`` if an exception was raised.
 
    .. note::