flag: document that Value.String must work on the zero value

Otherwise flag.PrintDefaults will fail when it tries to determine
whether the default is the zero value.

Fixes #16694.

Change-Id: I253fbf11ffc0a9069fd48c2c3cf3074df53e3a03
Reviewed-on: https://go-review.googlesource.com/27003
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/flag/flag.go

@@ -238,6 +238,7 @@ func (d *durationValue) String() string { return (*time.Duration)(d).String() }
 // rather than using the next command-line argument.
 //
 // Set is called once, in command line order, for each flag present.
+// String must work on the zero value of the dynamic type.
 type Value interface {
 	String() string
 	Set(string) error