flag: add a little more doc comment to Duration.

The only text that describes the accepted format is in the package doc,
which is far away from these functions. The other flag types don't need
this explicitness because they are more obvious.

LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/101550043

src/pkg/flag/flag.go

@@ -628,18 +628,21 @@ func Float64(name string, value float64, usage string) *float64 {
 
 // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
 // The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func (f *FlagSet) DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
 	f.Var(newDurationValue(value, p), name, usage)
 }
 
 // DurationVar defines a time.Duration flag with specified name, default value, and usage string.
 // The argument p points to a time.Duration variable in which to store the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func DurationVar(p *time.Duration, name string, value time.Duration, usage string) {
 	CommandLine.Var(newDurationValue(value, p), name, usage)
 }
 
 // Duration defines a time.Duration flag with specified name, default value, and usage string.
 // The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time.Duration {
 	p := new(time.Duration)
 	f.DurationVar(p, name, value, usage)
@@ -648,6 +651,7 @@ func (f *FlagSet) Duration(name string, value time.Duration, usage string) *time
 
 // Duration defines a time.Duration flag with specified name, default value, and usage string.
 // The return value is the address of a time.Duration variable that stores the value of the flag.
+// The flag accepts a value acceptable to time.ParseDuration.
 func Duration(name string, value time.Duration, usage string) *time.Duration {
 	return CommandLine.Duration(name, value, usage)
 }