Document the hover modes 'vline' and 'hline'

Support for these modes has been added a while ago but never documented.

It works by passing 'vline'/'hline' to `hover` or in the list of `tools`:

```python
import hvplot.pandas # noqa
from bokeh.sampledata.degrees import data as deg

deg.hvplot.line(x='Year', y=['Business', 'Computer Science'], hover='vline') # or tools=['vline']
```