Document `setViewportSize` relation with filmstrip capturing

[jakub-g]

https://github.com/WPO-Foundation/webpagetest-docs/blob/main/src/scripting.md#setviewportsize

I tried using setViewportSize in my script to capture enough of the page under test to see interesting things in filmstrip that are hidden behind cookie banner.

I noticed that if I set viewport too big, filmstrip is no longer captured; I guess to avoid huge videos which take a lot of storage.

I think it would be good to document what is the viewport threshold for that. From what I saw experimentally it's neither width or height alone, but the multiple of them? (i.e. number of pixels)