Update the proposal for UTCClock to reflect the discussion surrounding it

[phausler]

There were a few places where the examples left some lack of clarity of the proposed clock usages and behaviors. This refines those examples to a more concrete use case that avoids any confusion about the role associated with UTCClock and additionally clarifies the stance on the placement/homing of Date, the definition of timezones and the relationship to UTC, the naming of systemEpoch and finally a reflection of the proposal discussion out lasting the cutoff of 6.2.

[itingliu]

@swift-ci please test