[css-anchor-position-1] Clarify the nature of `@position-try` tree-scoped name

[dshin-moz]

From this WPT test, it seems that we're expecting @position-try name to be a tree-scoped name. Further reading the spec linked by the test, it seems that this tree-scoped name is global.

Ideally, we should reference this in the definition of @posiiton-try.