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

tv42/old-go,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,pombredanne/go-deleteme