[tz] [PROPOSED] Cite US DST legislation

[Paul Eggert]

walter harms wrote:
> i would only cite the current ruling here.

But tzdb's "Rule US" lines also cover historical data, and so rely on the older 
citations. The proposed comments don't summarize all the US DST legislation, 
only the parts that matter to tzdb's "Rule US" lines.

Besides, it seemed a little funny that tzdb already has extensive legal 
citations for countries ranging from Uruguay to the UK, while not bothering to 
provide them for the US.