gh-116938: Clarify documentation of dict and dict.update regarding the positional argument they accept

Doc/library/stdtypes.rst

@@ -4505,13 +4505,13 @@ can be used interchangeably to index the same dictionary entry.
      ``dict([('foo', 100), ('bar', 200)])``, ``dict(foo=100, bar=200)``
 
    If no positional argument is given, an empty dictionary is created.
-   If a positional argument is given and it is a mapping object, a dictionary
-   is created with the same key-value pairs as the mapping object.  Otherwise,
-   the positional argument must be an :term:`iterable` object.  Each item in
-   the iterable must itself be an iterable with exactly two objects.  The
-   first object of each item becomes a key in the new dictionary, and the
-   second object the corresponding value.  If a key occurs more than once, the
-   last value for that key becomes the corresponding value in the new
+   If a positional argument is given and it defines a ``keys()`` method, a
+   dictionary is created by calling ``__getitem__`` on the argument with each
+   returned key from the method.  Otherwise, the positional argument must be an
+   :term:`iterable` object.  Each item in the iterable must itself be an iterable
+   with exactly two objects.  The first object of each item becomes a key in the new
+   dictionary, and the second object the corresponding value.  If a key occurs more
+   than once, the last value for that key becomes the corresponding value in the new
    dictionary.
 
    If keyword arguments are given, the keyword arguments and their values are
@@ -4669,9 +4669,10 @@ can be used interchangeably to index the same dictionary entry.
       Update the dictionary with the key/value pairs from *other*, overwriting
       existing keys.  Return ``None``.
 
-      :meth:`update` accepts either another dictionary object or an iterable of
-      key/value pairs (as tuples or other iterables of length two).  If keyword
-      arguments are specified, the dictionary is then updated with those
+      :meth:`update` accepts either another object with a ``keys()`` method
+      (in which case ``__getitem__`` is called with every key returned from the method).
+      or an iterable of key/value pairs (as tuples or other iterables of length two).
+      If keyword arguments are specified, the dictionary is then updated with those
       key/value pairs: ``d.update(red=1, blue=2)``.
 
    .. method:: values()

Misc/NEWS.d/next/Documentation/2024-10-09-21-41-36.gh-issue-116938.ChLcRx.rst

@@ -0,0 +1,2 @@
+Clarify documentation of :class:`dict` and :meth:`dict.update` regarding the
+positional argument they accept.