time: document that calendrical calculations are with no leap seconds

Fixes #15247

Change-Id: I942fb2eacd1b54bab66cc147a6b047a3ffce0b84
Reviewed-on: https://go-review.googlesource.com/30595Reviewed-by: Russ Cox <rsc@golang.org>

src/time/time.go

@@ -4,7 +4,8 @@
 
 // Package time provides functionality for measuring and displaying time.
 //
-// The calendrical calculations always assume a Gregorian calendar.
+// The calendrical calculations always assume a Gregorian calendar, with
+// no leap seconds.
 package time
 
 import "errors"