Add migration note for removal of Document.resize()

[mattpap]

Apparently this was advertised and needs a replacement. This is a follow up on PR #5976.

[bryevdv]

@mattpap what is the status of this / what is the proper guidance for anyone that was using it?

[mattpap]

@bryevdv, I didn't write this yet. One should use view.resize() instead, or

for (var view_id in Bokeh.index) {
   Bokeh.index[view_id].resize()
}

[mattpap]

I added a note in PR #6384.

[github-actions]

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