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/Subscriptions.php

@@ -28,6 +28,7 @@ class Subscriptions extends BaseCollection
      *      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}
      *  } $attributes The subscription data:
      *    - group:          The group (optional)
@@ -43,6 +44,7 @@ class Subscriptions extends BaseCollection
      *    - trial_days:     A free trial of 1–365 days from the user's confirmation on (optional)
      *    - trial_ends_at:  Or a fixed end of the free trial (ISO 8601, in the future; optional)
      *    - trial_once:     Refuse the trial if the user already had one with the app in the group (optional)
+     *    - trial_payment_method: TrialPaymentMethod::REQUIRED (default) or ::OPTIONAL, only with a trial (optional)
      *    - discount:       A discount per billed period, see AppSubscriptions::create() (optional)
      * @throws Throwable
      */