message: docs: add note on payment intent
diff: //! println!("{:?}", charges); // => List { data: [Charge { id: "ch_12345", .. }] }
  //! # }
  //! ```
  +//!
  +//! > **A note about creating card tokens**: Stripe introduced the PaymentIntent api
  +//! > to replace the old token and charge API. This library only supports the former. To migrate, you can
  +//! > have a look at [the official migration guide](https://stripe.com/docs/payments/payment-intents/migration).
  #![allow(clippy::map_clone)]
  #![allow(clippy::large_enum_variant)]
  