Be explicit in the RSA documentation

# FIXME for 2023: return Optional[bytes] and return None on encryption failure?

def rsa_encrypt(key: rsa.RSAPublicKey, plaintext: bytes) -> bytes:

    """

    Uses RSA ``key`` to encrypt the ``plaintext``.

    Uses RSA public ``key`` to encrypt the ``plaintext``.

    :param key: The RSA public key to use for the encryption.

    :param plaintext: The plaintext bytes to be encrypted.