DOC: added references to argmin/argmin API changes to release notes/v0.13.0 relating to GH6214

Author: jreback

doc/source/release.rst

@@ -554,7 +554,9 @@ API Changes
     na values (:issue:`5521`)
   - The refactoring involving``Series`` deriving from ``NDFrame`` breaks ``rpy2<=2.3.8``. an Issue
     has been opened against rpy2 and a workaround is detailed in :issue:`5698`. Thanks @JanSchulz.
-
+  - ``Series.argmin`` and ``Series.argmax`` are now aliased to ``Series.idxmin`` and ``Series.idxmax``.
+    These return the *index* of the  min or max element respectively. Prior to 0.13.0 these would return
+    the position of the min / max element (:issue:`6214`)
 
 Internal Refactoring
 ~~~~~~~~~~~~~~~~~~~~

doc/source/v0.13.0.txt

@@ -171,6 +171,9 @@ API changes
 - ``Panel.reindex`` has the following call signature ``Panel.reindex(items=None, major_axis=None, minor_axis=None, **kwargs)``
    to conform with other ``NDFrame`` objects. See :ref:`Internal Refactoring<whatsnew_0130.refactoring>` for more information.
 
+- ``Series.argmin`` and ``Series.argmax`` are now aliased to ``Series.idxmin`` and ``Series.idxmax``. These return the *index* of the
+   min or max element respectively. Prior to 0.13.0 these would return the position of the min / max element. (:issue:`6214`)
+
 Prior Version Deprecations/Changes
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~