Update docs.

Author: serhiy-storchaka

Doc/c-api/arg.rst

@@ -339,8 +339,9 @@ Other objects
 
    .. versionadded:: 3.3
 
-``(items)`` (:class:`tuple`) [*matching-items*]
-   The object must be a Python sequence whose length is the number of format units
+``(items)`` (sequence) [*matching-items*]
+   The object must be a Python sequence (except :class:`str`, :class:`bytes`
+   or :class:`bytearray`) whose length is the number of format units
    in *items*.  The C arguments must correspond to the individual format units in
    *items*.  Format units for sequences may be nested.
 
@@ -351,6 +352,9 @@ Other objects
    The *converter* for the ``O&`` format unit in *items* must not store
    a borrowed buffer or a borrowed reference.
 
+   .. versionchanged:: next
+      :class:`str` and :class:`bytearray` objects no longer accepted as a sequence.
+
    .. deprecated:: next
       Non-tuple sequences are deprecated if *items* contains format units
       which store a borrowed buffer or a borrowed reference.