Complete the signature of plot methods with all their kind options

The converter class has a mapping from plot method name to options `_kind_options`.

```python
class HoloViewsConverter:
...
# Options specific to a particular plot type
_kind_options = {
...
'hexbin': ['C', 'reduce_function', 'gridsize', 'logz', 'min_count'],
...
}
```

These options do not all appear in the signature of the plot method exposed to users, for instance, `min_count` for `.hexbin()`.

```python
class hvPlotTabular(hvPlotBase):
...
def hexbin(self, x=None, y=None, C=None, colorbar=True, **kwds):
...
```

I'm not sure why some options aren't exposed in the signature, if there's one, I haven't found it yet.

Otherwise, I'd suggest adding:

- [ ] making sure the options *already defined in the signature* are correctly documented in the docstring
- [ ] add all the options not defined in the signature but in `_kind_options` to the signature.

In order to complete the second item, the default value of these options will have to be found and exposed. I assume they map to some HoloViews Parameter.