by argument not working for hist plot

I'm working on updating the docstrings for the `hist` plot. I would expect the `by` argument to work similarly to how it works for other hvPlot plots and/ or similarly to how it works for Pandas plot https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.DataFrame.plot.hist.html.


But it seems to have no effect.

## With `by`


```python
import hvplot.pandas # noqa
import pandas as pd
import numpy as np
age_list = [8, 10, 12, 14, 72, 74, 76, 78, 20, 25, 30, 35, 60, 85]
df = pd.DataFrame({"gender": list("MMMMMMMMFFFFFF"), "age": age_list})
df.hvplot.hist(y=["age"], by="gender")
```

## Without `by`


```python
import hvplot.pandas # noqa
import pandas as pd
import numpy as np
age_list = [8, 10, 12, 14, 72, 74, 76, 78, 20, 25, 30, 35, 60, 85]
df = pd.DataFrame({"gender": list("MMMMMMMMFFFFFF"), "age": age_list})
df.hvplot.hist(y=["age"])
```