message: Add ciphertext_destination write preconditions to docstring.
diff: @@ -35,6 +35,7 @@ class StreamingAead(object):
  @abc.abstractmethod
  def new_encrypting_stream(self, ciphertext_destination: BinaryIO,
  associated_data: bytes) -> BinaryIO:
  + # pyformat: disable
  """Get a new encrypting stream that writes to ciphertext_destination.
  Args:
  @@ -45,6 +46,17 @@ class StreamingAead(object):
  not included in the ciphertext and must be passed in as a parameter for
  decryption.
  + The ciphertext_destination's write() method is expected to present one of
  + the following three behaviours in the case of a partial or failed write():
  + - return a non-negative integer number of bytes written
  + - return None (equivalent to returning 0)
  + - raise BlockingIOError with characters_written set correctly to a
  + non-negative integer (equivalent to returning that integer)
  + In the case of a full write, the number of bytes written should be returned.
  +
  + The standard BufferedIOBase and RawIOBase base classes exhibit these
  + behaviours and are hence supported.
  +
  Returns:
  An encrypting file object wrapper around 'ciphertext_destination', such
  that any bytes written to the wrapper are AEAD-encrypted using
  @@ -67,6 +79,7 @@ class StreamingAead(object):
  Raises:
  tink.TinkError if the creation fails.
  """
  + # pyformat: enable
  raise NotImplementedError()
  @abc.abstractmethod
  