[css-scroll-start] Call out interaction with scroll anchors

[flackr]

When a scroll-start position is specified, it should be treated as the default initial scroll position. However, if a scroll anchor or other mechanism has a target it should be able to override the scroll-start. We should call this out in the explainer as another advantage.

[argyleink]

lemme know if you dont think i covered it well in the explainer!

https://github.com/argyleink/ScrollSnapExplainers/blob/main/css-scroll-start/explainer.md#interactions-with-fragment-navigation

[flackr]

Looks good, thanks!