Enable me to get more information about the different options in the explorer

# Request

Add tooltips to the widgets, a tab with the docstring for the `kind` of plot and/ or a link to the reference documentation of the `kind` of plot.

## Motivation

The `explorer` makes it much, much easier to understand the available parameters of `hvplot.kind_of_plot`. But users might still not know what `Dynspread` means. Or the difference between `Datashade` and `Rasterize`. It would be very helpful if the widgets had tooltips explaining this.

As providing tooltips in HoloViz is not easy, a second best solution might be to provide the (nicely formatted) docstring of the `kind` of plot on a separate tab. Or just a link next to the reference guide for the `kind` of plot.

That would reduce the friction of using hvPlot quite a lot I believe.