flag: Describe the ErrorHandling enum values.

ContinueOnError is particularly confusing, because it causes
FlagSet.Parse to return as soon as it sees an error. I gather that the
intent is "continue the program" rather than "continue parsing",
compared to exiting or panicking.

Change-Id: I27370ce1f321ea4debcee5b03faff3532495c71a
Reviewed-on: https://go-review.googlesource.com/10740Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -249,13 +249,14 @@ type Getter interface {
 	Get() interface{}
 }
 
-// ErrorHandling defines how to handle flag parsing errors.
+// ErrorHandling defines how FlagSet.Parse behaves if the parse fails.
 type ErrorHandling int
 
+// These constants cause FlagSet.Parse to behave as described if the parse fails.
 const (
-	ContinueOnError ErrorHandling = iota
-	ExitOnError
-	PanicOnError
+	ContinueOnError ErrorHandling = iota // Return a descriptive error.
+	ExitOnError                          // Call os.Exit(2).
+	PanicOnError                         // Call panic with a descriptive error.
 )
 
 // A FlagSet represents a set of defined flags.  The zero value of a FlagSet