Revert "Raise a RuntimeError when tee iterator is consumed from different threads (GH-15567)" (GH-15736)

This reverts commit fa220ec7633e9674baccc28dde987f29d7f65141.
diff --git a/Doc/library/itertools.rst b/Doc/library/itertools.rst
index 8e7899e..a3f403a 100644
--- a/Doc/library/itertools.rst
+++ b/Doc/library/itertools.rst
@@ -643,8 +643,7 @@
 
    Once :func:`tee` has made a split, the original *iterable* should not be
    used anywhere else; otherwise, the *iterable* could get advanced without
-   the tee objects being informed. the :func:`tee` iterator can not be consumed
-   from different threads, even if an underlying iterator is thread-safe.
+   the tee objects being informed.
 
    This itertool may require significant auxiliary storage (depending on how
    much temporary data needs to be stored). In general, if one iterator uses