gh-57095: Add note about input splitting in datetime.*.strptime

Doc/library/datetime.rst

@@ -2689,6 +2689,11 @@ 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, ``%Y%m%d`` parses the string ``2026111`` as ``2026-11-01``,
+   not ``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