time: improve Truncate and Round documentation

Truncate and Round operate on absolute time, which means that
Truncate(Hour) may return a time with non-zero Minute(). Document that
more clearly, and remove the misleading example which suggests it is
safe.

Updates #16647

Change-Id: I930584ca030dd12849195d45e49ed2fb74e0c9ac
Reviewed-on: https://go-review.googlesource.com/28730Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/time/example_test.go

@@ -251,20 +251,18 @@ func ExampleTime_Truncate() {
 		2 * time.Second,
 		time.Minute,
 		10 * time.Minute,
-		time.Hour,
 	}
 
 	for _, d := range trunc {
-		fmt.Printf("t.Truncate(%6s) = %s\n", d, t.Truncate(d).Format("15:04:05.999999999"))
+		fmt.Printf("t.Truncate(%5s) = %s\n", d, t.Truncate(d).Format("15:04:05.999999999"))
 	}
 
 	// Output:
-	// t.Truncate(   1ns) = 12:15:30.918273645
-	// t.Truncate(   1µs) = 12:15:30.918273
-	// t.Truncate(   1ms) = 12:15:30.918
-	// t.Truncate(    1s) = 12:15:30
-	// t.Truncate(    2s) = 12:15:30
-	// t.Truncate(  1m0s) = 12:15:00
-	// t.Truncate( 10m0s) = 12:10:00
-	// t.Truncate(1h0m0s) = 12:00:00
+	// t.Truncate(  1ns) = 12:15:30.918273645
+	// t.Truncate(  1µs) = 12:15:30.918273
+	// t.Truncate(  1ms) = 12:15:30.918
+	// t.Truncate(   1s) = 12:15:30
+	// t.Truncate(   2s) = 12:15:30
+	// t.Truncate( 1m0s) = 12:15:00
+	// t.Truncate(10m0s) = 12:10:00
 }

src/time/time.go

@@ -1103,6 +1103,11 @@ func Date(year int, month Month, day, hour, min, sec, nsec int, loc *Location) T
 
 // Truncate returns the result of rounding t down to a multiple of d (since the zero time).
 // If d <= 0, Truncate returns t unchanged.
+//
+// Truncate operates on the time as an absolute duration since the
+// zero time; it does not operate on the presentation form of the
+// time. Thus, Truncate(Hour) may return a time with a non-zero
+// minute, depending on the time's Location.
 func (t Time) Truncate(d Duration) Time {
 	if d <= 0 {
 		return t
@@ -1114,6 +1119,11 @@ func (t Time) Truncate(d Duration) Time {
 // Round returns the result of rounding t to the nearest multiple of d (since the zero time).
 // The rounding behavior for halfway values is to round up.
 // If d <= 0, Round returns t unchanged.
+//
+// Round operates on the time as an absolute duration since the
+// zero time; it does not operate on the presentation form of the
+// time. Thus, Round(Hour) may return a time with a non-zero
+// minute, depending on the time's Location.
 func (t Time) Round(d Duration) Time {
 	if d <= 0 {
 		return t