Update backward incompatible section in v1.1.0 relase

Author: PuneethaPai

doc/source/whatsnew/v1.1.0.rst

@@ -342,6 +342,7 @@ Backwards incompatible API changes
 - :meth:`Series.to_timestamp` now raises a ``TypeError`` if the axis is not a :class:`PeriodIndex`. Previously an ``AttributeError`` was raised (:issue:`33327`)
 - :meth:`Series.to_period` now raises a ``TypeError`` if the axis is not a :class:`DatetimeIndex`. Previously an ``AttributeError`` was raised (:issue:`33327`)
 - :func: `pandas.api.dtypes.is_string_dtype` no longer incorrectly identifies categorical series as string.
+- :func: `pandas.core.reshape.merge` now checks ``suffixes`` parameter type to be ``tuple`` and raises ``TypeError``, whereas before a ``list`` or ``set`` were accepted and that the ``set`` could produce unexpected results (:issue:`33740`)
 
 ``MultiIndex.get_indexer`` interprets `method` argument differently
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^