DOC: Show warning as a comment (#20329)

Author: TomAugspurger

pandas/errors/__init__.py

@@ -68,8 +68,7 @@ class DtypeWarning(Warning):
     ...                    'b': ['b'] * 300000})
     >>> df.to_csv('test.csv', index=False)
     >>> df2 = pd.read_csv('test.csv')
-
-        DtypeWarning: Columns (0) have mixed types
+    ... # DtypeWarning: Columns (0) have mixed types
 
     Important to notice that ``df2`` will contain both `str` and `int` for the
     same input, '1'.