Clarify whatsnew

Author: Matt Roeschke

doc/source/whatsnew/v0.25.0.rst

@@ -562,7 +562,7 @@ Other API changes
 - Removed support of gtk package for clipboards (:issue:`26563`)
 - Using an unsupported version of Beautiful Soup 4 will now raise an ``ImportError`` instead of a ``ValueError`` (:issue:`27063`)
 - :meth:`Series.to_excel` and :meth:`DataFrame.to_excel` will now raise a ``ValueError`` when saving timezone aware data. (:issue:`27008`, :issue:`7056`)
-- :meth:`DataFrame.to_hdf` and :meth:`Series.to_hdf` will now raise a ``NotImplementedError`` when saving a :class:`MultiIndex` with extention data types (:issue:`7775`)
+- :meth:`DataFrame.to_hdf` and :meth:`Series.to_hdf` will now raise a ``NotImplementedError`` when saving a :class:`MultiIndex` with extention data types for a ``fixed`` format. (:issue:`7775`)
 
 .. _whatsnew_0250.deprecations: