flag: clarify that a flag cannot be re-defined

Fixes #31694

Change-Id: Ifb2ad2dc41c449668c0f6a4d4cfb9b583e5591f2
Reviewed-on: https://go-review.googlesource.com/c/go/+/206126
Reviewed-by: Rob Pike <r@golang.org>

flag.go

@@ -314,6 +314,9 @@ const (
 
 // A FlagSet represents a set of defined flags. The zero value of a FlagSet
 // has no name and has ContinueOnError error handling.
+//
+// Flag names must be unique within a FlagSet. An attempt to define a flag whose
+// name is already in use will cause a panic.
 type FlagSet struct {
 	// Usage is the function called when an error occurs while parsing flags.
 	// The field is a function (not a method) that may be changed to point to