DOC: clean-up 0.21.0 whatsnew file (#18001)

* DOC: clean-up 0.21.0 whatsnew file

* literal include warning

* split highlight

* reorder subsections in API changes (somewhat more in order of importance)

* move python 3.4 support drop to section dropped version support of deps

* minor formatting

* add indexing with list of partly missing labels to highlights

* udpate parquet explanation

* update highlights in release notes

* format as subsubsections

* wording

doc/source/release.rst

@@ -49,11 +49,14 @@ version.
 
 Highlights include:
 
-- Integration with `Apache Parquet <https://parquet.apache.org/>`__, including a new top-level :func:`read_parquet` and :func:`DataFrame.to_parquet` method, see :ref:`here <io.parquet>`.
+- Integration with `Apache Parquet <https://parquet.apache.org/>`__, including a new top-level :func:`read_parquet` function and :meth:`DataFrame.to_parquet` method, see :ref:`here <whatsnew_0210.enhancements.parquet>`.
 - New user-facing :class:`pandas.api.types.CategoricalDtype` for specifying
   categoricals independent of the data, see :ref:`here <whatsnew_0210.enhancements.categorical_dtype>`.
-- The behavior of ``sum`` and ``prod`` on all-NaN Series/DataFrames is now consistent and no longer depends on whether `bottleneck <http://berkeleyanalytics.com/bottleneck>`__ is installed, see :ref:`here <whatsnew_0210.api_breaking.bottleneck>`
+- The behavior of ``sum`` and ``prod`` on all-NaN Series/DataFrames is now consistent and no longer depends on whether `bottleneck <http://berkeleyanalytics.com/bottleneck>`__ is installed, see :ref:`here <whatsnew_0210.api_breaking.bottleneck>`.
 - Compatibility fixes for pypy, see :ref:`here <whatsnew_0210.pypy>`.
+- Additions to the ``drop``, ``reindex`` and ``rename`` API to make them more consistent, see :ref:`here <whatsnew_0210.enhancements.drop_api>`.
+- Addition of the new methods ``DataFrame.infer_objects`` (see :ref:`here <whatsnew_0210.enhancements.infer_objects>`) and ``GroupBy.pipe`` (see :ref:`here <whatsnew_0210.enhancements.GroupBy_pipe>`).
+- Indexing with a list of labels, where one or more of the labels is missing, is deprecated and will raise a KeyError in a future version, see :ref:`here <whatsnew_0210.api_breaking.loc>`.
 
 See the :ref:`v0.21.0 Whatsnew <whatsnew_0210>` overview for an extensive list
 of all enhancements and bugs that have been fixed in 0.21.0