explain `initialDiaglog` usage

[mydiemho]

this is how componentDialog determine entry point

const DONATION_DIALOG = 'donationDialog';

class DonateFoodDialog extends ComponentDialog {
    constructor(dialogId) {
        super(dialogId);

        // entry point of the component, THIS IS REQUIRED
        this.initialDialogId = DONATION_DIALOG;

        this.addDialog(new ChoicePrompt('choicePrompt'));

        // Define the conversation flow using a waterfall model.
        this.addDialog(new WaterfallDialog(DONATION_DIALOG, [
            async function (step) {