gh-57095: Add note about input splitting in datetime.*.strptime (GH-131049)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: StanFromIreland

Doc/library/datetime.rst

@@ -2689,6 +2689,12 @@ For the :meth:`.datetime.strptime` and :meth:`.date.strptime` class methods,
 the default value is ``1900-01-01T00:00:00.000``: any components not specified
 in the format string will be pulled from the default value.
 
+.. note::
+   Format strings without separators can be ambiguous for parsing. For
+   example, with ``%Y%m%d``, the string ``2026111`` may be parsed either as
+   ``2026-11-01`` or as ``2026-01-11``.
+   Use separators to ensure the input is parsed as intended.
+
 .. note::
    When used to parse partial dates lacking a year, :meth:`.datetime.strptime`
    and :meth:`.date.strptime` will raise when encountering February 29 because