Document trials without a payment method

trial_payment_method (TrialPaymentMethod::REQUIRED, the default, or
::OPTIONAL) on app subscriptions, subscriptions and checkout sessions:
an optional trial can be confirmed without a payment method and is
cancelled at its end if there is still none (cancellation_reason and
subscription.cancelled data.reason
CancellationReason::TRIAL_ENDED_WITHOUT_PAYMENT_METHOD).
subscription.trial_will_end carries data.payment_method_missing.

src/Id/Enums/TrialPaymentMethod.php

@@ -0,0 +1,20 @@
+<?php
+
+namespace Anikeen\Id\Enums;
+
+/**
+ * `trial_payment_method` of a subscription with a free trial: whether the user needs a payment
+ * method to confirm it. Null on subscriptions without a trial.
+ */
+class TrialPaymentMethod
+{
+    /** A payment method is needed to confirm the trial (the default). */
+    const REQUIRED = 'required';
+
+    /**
+     * The trial can be confirmed without a payment method (if nothing else has to be paid). If the
+     * user still has none when the trial ends, the subscription is cancelled then instead of billed
+     * (`cancellation_reason` CancellationReason::TRIAL_ENDED_WITHOUT_PAYMENT_METHOD).
+     */
+    const OPTIONAL = 'optional';
+}