Include time-variable examples in Accessibility API docs…

The Accessibility API docs page at:
https://openusd.org/release/user_guides/schemas/usdUI/AccessibilityAPI.html

…doesn't include an example as to how you'd make a time-variable label, such as this one for a street crossing sign.

string accessibility:default:label.timeSamples = {0: "Walk", 10: "Don't Walk"}

It'd be great to add that or another time varied example.