Move the entry to API change section and make it prominent

Author: makbigc

doc/source/whatsnew/v1.0.0.rst

@@ -39,6 +39,28 @@ Backwards incompatible API changes
 - :class:`pandas.core.groupby.GroupBy.transform` now raises on invalid operation names (:issue:`27489`).
 -
 
+.. _whatsnew_0250.api_breaking.series_string_conversion:
+
+String conversion of Series with nan
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+ When converting into string, :meth:`Series.astype` will not convert ``np.nan`` into string and keep it as missing value (:issue:`25353`)
+
+ *pandas 0.25.x*
+
+.. code-block:: ipython
+
+   In [1]: pd.Series(['foo', np.nan]).astype(str)
+   Out[2]:
+   0    foo
+   1    nan
+   dtype: object
+
+*pandas 1.0.0*
+
+.. ipython:: python
+   pd.Series(['foo', np.nan]).astype(str)
+
 .. _whatsnew_1000.api.other:
 
 Other API changes
@@ -150,7 +172,7 @@ Indexing
 Missing
 ^^^^^^^
 
-- When converting into string, :meth:`Series.astype` will keep ``np.nan`` as missing value (:issue:`25353`)
+-
 -
 
 MultiIndex