Provide a cross-reference to PEP 798 unpacking syntax (#152454)

Co-authored-by: Rishit Agnihotri <Twix1288@users.noreply.github.com>

Author: rhettinger

Doc/library/itertools.rst

@@ -197,6 +197,10 @@ loops that truncate the stream.
           for iterable in iterables:
               yield from iterable
 
+   Note that :pep:`798` unpacking syntax provides similar functionality
+   so that ``list(chain(p, q))`` could be written as
+   ``[*s for s in (p, q)]``.
+
 
 .. classmethod:: chain.from_iterable(iterable)
 
@@ -208,6 +212,10 @@ loops that truncate the stream.
           for iterable in iterables:
               yield from iterable
 
+   Note that :pep:`798` unpacking syntax provides similar functionality
+   so that ``list(chain.from_iterable(iterables))`` could be written as
+   ``[*s for s in iterables]``.
+
 
 .. function:: combinations(iterable, r)