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/Subscription.php

@@ -27,6 +27,8 @@ use Throwable;
  * @property string $ends_at
  * @property ?string $cancels_at when a scheduled cancellation takes effect
  * @property ?string $trial_ends_at
+ * @property ?string $trial_payment_method required or optional with a trial (TrialPaymentMethod), null without
+ * @property ?string $cancellation_reason why the billing cancelled it (CancellationReason), null otherwise
  * @property ?object $discount {label, percent_off, amount_off, duration, duration_in_periods, periods_remaining}; periods_remaining is null for `forever`, 0 once it ended
  * @property null|string $confirmation_url where the user confirms it (third-party apps)
  */