DOC: Add 1.5.x note to main (#50822)

Author: phofl

doc/source/whatsnew/v1.5.3.rst

@@ -47,6 +47,7 @@ Other
     as pandas works toward compatibility with SQLAlchemy 2.0.
 
 - Reverted deprecation (:issue:`45324`) of behavior of :meth:`Series.__getitem__` and :meth:`Series.__setitem__` slicing with an integer :class:`Index`; this will remain positional (:issue:`49612`)
+- A ``FutureWarning`` raised when attempting to set values inplace with :meth:`DataFrame.loc` or :meth:`DataFrame.iloc` has been changed to a ``DeprecationWarning`` (:issue:`48673`)
 
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 .. _whatsnew_153.contributors: