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

[maximlt]

xarray defaults to plotting 2D DataArrays with xarray.plot.pcolormesh (link):

We use xarray.plot.pcolormesh() as the default two-dimensional plot method because it is more flexible than 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.

[jbednar]

Sounds good. I think the current behavior is safe since I believe it will complain if it the grid is not regularly spaced.