BUG: Read_Table and Read_Csv does not raise when delim_whitespace=True and sep=default is given
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
import io
f = io.StringIO("a  b  c\n1 -2 -3\n4  5   6")
df = pd.read_table(f, sep="\t", delim_whitespace=True)
df = pd.read_csv(f, sep=",", delim_whitespace=True)

```

#### Problem description

Both statement should raise a ``ValueError``

#### Expected Output

``ValueError``

Must also edit docs. See #36560 for discussion.

cc @simonjayhawkins 

#### Output of ``pd.show_versions()``

<details>

master

</details>
