re-add deleted comment, uncomment previously commented line

Author: rforgione

pandas/core/frame.py

@@ -2196,6 +2196,10 @@ def eval(self, expr, inplace=True, **kwargs):
             If the expression contains an assignment, whether to return a new
             DataFrame or mutate the existing.
 
+            WARNING: inplace=None currently falls back to to True, but
+            in a future version, will default to False.  Use inplace=True
+            explicitly rather than relying on the default.
+
             .. versionadded:: 0.18.0
 
         kwargs : dict
@@ -3258,7 +3262,7 @@ def sort(self, columns=None, axis=0, ascending=True, inplace=False,
         -------
         sorted : DataFrame
         """
-        # nv.validate_sort(tuple(), kwargs)
+        nv.validate_sort(tuple(), kwargs)
 
         if columns is None:
             warnings.warn("sort(....) is deprecated, use sort_index(.....)",