flag: document use of FlagSet name parameter

Fixes #24225

Change-Id: I876ac1b9d3615283f7b88cfa0b965ef81a57f056
Reviewed-on: https://go-review.googlesource.com/102955Reviewed-by: Rob Pike <r@golang.org>

src/flag/flag.go

@@ -981,7 +981,8 @@ func commandLineUsage() {
 }
 
 // NewFlagSet returns a new, empty flag set with the specified name and
-// error handling property.
+// error handling property. If the name is not empty, it will be printed
+// in the default usage message and in error messages.
 func NewFlagSet(name string, errorHandling ErrorHandling) *FlagSet {
 	f := &FlagSet{
 		name:          name,