Auto merge of #29784 - mrobinson:compositor-side-scroll-tree-cleanup-send-display-list, r=atbrakhi

Clean up and document the `send_display_list` interface

This moves more members to the CompositorDisplayListInfo struct, which now holds all miscellaneous, non-WebRender data when sending display lists. It also documents what each things sent with a display list does.

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- [x] These changes do not require tests because they do not change behavior.

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