time: correct unrepresentable Unix time comment

It's easy for someone who wants a time bigger than any
valid time to reach for time.Unix(1<<63-1, 0), so it
makes sense to explicit say such value is not valid.

Fixes #10906 (again).

Change-Id: If71e32472ae40d86c30e629b982406040a73c4c7
Reviewed-on: https://go-review.googlesource.com/10266Reviewed-by: Russ Cox <rsc@golang.org>

src/time/time.go

@@ -966,8 +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.
+// Not all sec values have a corresponding time value. One such
+// value is 1<<63-1 (the largest int64 value).
 func Unix(sec int64, nsec int64) Time {
 	if nsec < 0 || nsec >= 1e9 {
 		n := nsec / 1e9