DOC: doc/source/indexing.rst says pd.df.ix is deprecated, show warning in generated doc. (#19596)

Author: xpvpc

pandas/core/indexing.py

@@ -1300,6 +1300,9 @@ class _IXIndexer(_NDFrameIndexer):
     """A primarily label-location based indexer, with integer position
     fallback.
 
+    Warning: Starting in 0.20.0, the .ix indexer is deprecated, in
+    favor of the more strict .iloc and .loc indexers.
+
     ``.ix[]`` supports mixed integer and label based access. It is
     primarily label based, but will fall back to integer positional
     access unless the corresponding axis is of integer type.