time: add comment explaining rejection of years outside [0000,9999]
time: add comment explaining rejection of years outside [0000,9999]

This has come up twice now. Redirect future questions
to the explanation in the issue tracker.

LGTM=iant, r
R=r, iant
CC=golang-codereviews
https://codereview.appspot.com/79550043

apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang,apparentlymart/golang