time: document that not all Unix time can be represented

Fixes #10906.

Change-Id: I7ae25a500df493c1e78183d69d89b3e2a64a0d1a
Reviewed-on: https://go-review.googlesource.com/10223Reviewed-by: Andrew Gerrand <adg@golang.org>

src/time/time.go

@@ -966,6 +966,8 @@ func (t *Time) UnmarshalText(data []byte) (err error) {
 // Unix returns the local Time corresponding to the given Unix time,
 // sec seconds and nsec nanoseconds since January 1, 1970 UTC.
 // It is valid to pass nsec outside the range [0, 999999999].
+// Not all sec values have a corresponding time value. Notable such
+// values are -1<<63 and 1<<63-1.
 func Unix(sec int64, nsec int64) Time {
 	if nsec < 0 || nsec >= 1e9 {
 		n := nsec / 1e9