Add sparse reshaping docs section

Author: kernc

doc/source/sparse.rst

@@ -181,6 +181,15 @@ the correct dense result.
    np.abs(arr)
    np.abs(arr).to_dense()
 
+.. _sparse.reshaping:
+
+Reshaping by stacking and unstacking
+------------------------------------
+
+``SparseSeries`` and ``SparseDataFrame`` implement ``.stack()`` and
+``.unstack()`` methods that should behave exactly the same as on
+:ref:`their non-sparse counterparts <reshaping.stacking>`.
+
 .. _sparse.scipysparse:
 
 Interaction with scipy.sparse

doc/source/whatsnew/v0.21.0.txt

@@ -303,7 +303,7 @@ Performance Improvements
 Bug Fixes
 ~~~~~~~~~
 
-- Bug in :func:`SparseSeries.unstack` and :func:`SparseDataFrame.stack` (:issue:`16614`, :issue:`15045`)
+- Bug in :func:`SparseSeries.unstack` and :func:`SparseDataFrame.stack`. See :ref:`docs <sparse.reshaping>` (:issue:`16614`, :issue:`15045`)
 
 Conversion
 ^^^^^^^^^^