time: add a comment about how to use the Duration constants

R=golang-dev, bradfitz, r, dsymonds
CC=golang-dev
https://golang.org/cl/5696078

src/pkg/time/time.go

@@ -384,6 +384,15 @@ type Duration int64
 
 // Common durations.  There is no definition for units of Day or larger
 // to avoid confusion across daylight savings time zone transitions.
+//
+// To count the number of units in a Duration, divide:
+//	second := time.Second
+//	fmt.Print(int64(second/time.Millisecond)) // prints 1000
+//
+// To convert an integer number of units to a Duration, multiply:
+//	seconds := 10
+//	fmt.Print(time.Duration(seconds)*time.Second) // prints 10s
+//
 const (
 	Nanosecond  Duration = 1
 	Microsecond          = 1000 * Nanosecond