Document that hvPlot defaults to plotting 2D DataArrays with an Image and not a QuadMesh unlike xarray

xarray defaults to plotting 2D DataArrays with `xarray.plot.pcolormesh` ([link](https://docs.xarray.dev/en/stable/user-guide/plotting.html#id2)):

> We use [xarray.plot.pcolormesh()](https://docs.xarray.dev/en/stable/generated/xarray.plot.pcolormesh.html#xarray.plot.pcolormesh) as the default two-dimensional plot method because it is more flexible than [xarray.plot.imshow()](https://docs.xarray.dev/en/stable/generated/xarray.plot.imshow.html#xarray.plot.imshow). However, for large arrays, imshow can be much faster than pcolormesh. If speed is important to you and you are plotting a regular mesh, consider using imshow.

hvPlot defaults to plotting in `Image` element, not a `QuadMesh`. This should be emphasized in the documentation.