[3.13] gh-133986: Document string split algorithm when sep is None and maxsplit is 0 (GH-133987) (#133992)

gh-133986: Document string split algorithm when sep is None and maxsplit is 0 (GH-133987)
---------
(cherry picked from commit 3e23047)

Co-authored-by: Joey Smith <joeysmith@gmail.com>
Co-authored-by: Ned Batchelder <ned@nedbatchelder.com>
Co-authored-by: Semyon Moroz <donbarbos@proton.me>

Author: 4 people

Doc/library/stdtypes.rst

@@ -2133,6 +2133,18 @@ expression support in the :mod:`re` module).
       >>> '   1   2   3   '.split()
       ['1', '2', '3']
 
+   If *sep* is not specified or is ``None`` and  *maxsplit* is ``0``, only
+   leading runs of consecutive whitespace are considered.
+
+   For example::
+
+      >>> "".split(None, 0)
+      []
+      >>> "   ".split(None, 0)
+      []
+      >>> "   foo   ".split(maxsplit=0)
+      ['foo   ']
+
 
 .. index::
    single: universal newlines; str.splitlines method