time: document when UnixNano overflows

[dsnet]

When I use the UnixNano method, I have to do some math to discover that the value will overflow in the year 2262 (I never seem to remember this). While 2262 is very far out for most applications, perhaps we should document that is the maximum representable date as an int64 since Unix epoch?

I dont feel a need to do a similar statement in the Unix method since the overflow date for that is beyond anything reasonable.

[gopherbot]

CL https://golang.org/cl/28478 mentions this issue.