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/Resources/AppCheckoutSessions.php

@@ -40,6 +40,7 @@ class AppCheckoutSessions
      *         trial_days?: null|int,
      *         trial_ends_at?: null|string,
      *         trial_once?: bool,
+     *         trial_payment_method?: null|string,
      *         discount?: null|array{label: string, percent_off?: float|string, amount_off?: float|string, duration: string, duration_in_periods?: int}
      *     }>,
      *     return_url?: null|string,
@@ -51,6 +52,8 @@ class AppCheckoutSessions
      *               repeated idempotency_key returns the first session instead of a new one.
      *               trial_days / trial_ends_at give a subscription a free trial from the user's
      *               confirmation on; trial_once is allowed on one subscription per group.
+     *               trial_payment_method `optional`: the session can be confirmed without a
+     *               payment method only if all of its subscriptions are such trials.
      *               discount: see AppSubscriptions::create(); the confirmation page shows it.
      *
      * @throws ApiException 422 validation (e.g. return_url), 409 "idempotency_key_conflict"