annotate required flags for bash_completions

some commands require that certain flags be set in order to function.
Annotate those flags so the bash completions will suggest to the user
that they be set.

pkg/kubectl/cmd/portforward.go

@@ -54,6 +54,7 @@ func NewCmdPortForward(f *cmdutil.Factory) *cobra.Command {
 		},
 	}
 	cmd.Flags().StringP("pod", "p", "", "Pod name")
+	cmd.MarkFlagRequired("pod")
 	// TODO support UID
 	return cmd
 }