Document rasterize=True together with by

[maximlt]

Document rasterize=True together with by, following the support added in #1132. This was not documented at the time as HoloViews wasn't yet released with the feature.

[maximlt]

@hoxbro @ahuang11 I haven't followed very closely the development of ImageStack in HoloViews and its ramification in Bokeh. Are there enough features released (even in dev versions of HoloViews) to create a nice and useful example with rasterize+by?

[philippjfr]

Yes, you should be able to create nice examples at this point. The only problem now are the hover tooltips.

[maximlt]

Ok so let's do that for this release.

[hoxbro]

We still need an official HoloViews 1.18 release before it makes sense.

[maximlt]

Ah true, well then it'll have to wait!