DOC: Add SparseArray.all and SparseArray.any to whatsnew note

Author: Licht-T

doc/source/whatsnew/v0.21.0.txt

@@ -544,6 +544,7 @@ Sparse
 
 - Bug in ``SparseSeries`` raises ``AttributeError`` when a dictionary is passed in as data (:issue:`16905`)
 - Bug in :func:`SparseDataFrame.fillna` not filling all NaNs when frame was instantiated from SciPy sparse matrix (:issue:`16112`)
+- Bug in :func:`SparseArray.all` and :func:`SparseArray.any` which are used but not implemented (:issue:`17570`)
 
 
 Reshaping