clarified the documentation for DF.drop_duplicates (pandas-dev#25056)

Author: mjmdavis

pandas/core/frame.py

@@ -4625,7 +4625,8 @@ def dropna(self, axis=0, how='any', thresh=None, subset=None,
     def drop_duplicates(self, subset=None, keep='first', inplace=False):
         """
         Return DataFrame with duplicate rows removed, optionally only
-        considering certain columns.
+        considering certain columns. Indexes, including time indexes
+        are ignored.
 
         Parameters
         ----------