Scoped label display and documentation tweaks (#23430)

Scoped label display and documentation tweaks (#23430)

* Fix scoped label left and right part breaking across lines.
* Remove slanted divider in scoped label display, make it straight.
After using this for a while, this feels more visually noisy than
helpful.
* Reduce contrast between scope and item to reduce probability of
unreadable text on background.
* Change documentation to remove mention of non-exclusive scoped labels.

Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>