DOC: replaces silent error for setattr on nonexistent name with UserWarning

Author: deniederhut

doc/source/indexing.rst

@@ -228,7 +228,7 @@ as an attribute:
    panel.one
 
 You can use attribute access to modify an existing element of a Series or column of a DataFrame, but be careful;
-if you try to use attribute access to create a new column, it fails silently, creating a new attribute rather than a
+if you try to use attribute access to create a new column, it issues a `UserWarning` and creates a new attribute rather than a
 new column.
 
 .. ipython:: python