Mark runc features experimental

Follow-up to PR 3296

Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>

features.go

@@ -20,6 +20,7 @@ var featuresCommand = cli.Command{
 	Description: `Show the enabled features.
    The result is parsable as a JSON.
    See https://pkg.go.dev/github.com/opencontainers/runc/types/features for the type definition.
+   The types are experimental and subject to change.
 `,
 	Action: func(context *cli.Context) error {
 		if err := checkArgs(context, 0, exactArgs); err != nil {