[css-grid-3] For masonry, clarify interactions between fragmenting and alignment (per fragment, not just track)

[jonjohnjohnson]

https://drafts.csswg.org/css-grid-3/#tracks-alignment
https://drafts.csswg.org/css-grid-3/#pagination

For masonry layout, if fragmented, can align-tracks be leveraged as much as possible within individual fragments instead of only per track within the first fragment? As I play with @MatsPalmgren demos linked in #4650 (comment) I realize align-tracks becomes less useful/intuitive once fragmented and honestly something I'd describe as "broken" (even within the first fragment) when using values such as center/end/space-around/space-evenly/stretch.

I know a major caveat is that individual grid items can also be fragmented, splitting their rendering across multiple fragments (ie not setting break-inside:avoid), leaving values such as or stretch or space-around difficult to imagine applying.