Extend documentation on thread safety

[fkohl04]

Hey all,

my Team is using the Tink library as described in the Google Developer portal. The example code creates a fresh PaymentTokenRecipient to unseal an encrypted String:

String decryptedMessage =
    new PaymentMethodTokenRecipient.Builder()
    .fetchSenderVerifyingKeysWith(
        GooglePaymentsPublicKeysManager.INSTANCE_PRODUCTION)
    .recipientId("merchant:[YOUR MERCHANT ID]")
    // This guide applies only to protocolVersion = ECv2
    .protocolVersion("ECv2")
    // Multiple private keys can be added to support graceful
    // key rotations.
    .addRecipientPrivateKey(PrivateKey1)
    .addRecipientPrivateKey(PrivateKey2)
    .build()
    .unseal(encryptedMessage);

Is the implementation of PaymentMethodTokenRecipient thread safe so that it would be possible to reuse an instance and especially use one instance from multiple threads?