Update UAX42 to document that common Unihan attributes are grouped starting with Unicode 17

[jowilco]

Common Unihan attributes, unlike non-Unihan attributes, were being applied at each character rather than the parent group element when generated using the previous UCDXML utility. From Unicode 17, common Unihan attributes will be applied to the group.

[markusicu]

I added this to the Unicode Tools dashboard. We have not paid a lot of attention to this dashboard and to the issues here; might be more successful as a PAG issue -- please try to keep track of it yourself.