message: Add ciphertext_source read() conditions to docstring.
diff: @@ -93,6 +93,14 @@ class StreamingAead(object):
  associated_data: Associated data to be used by the AEAD decryption. It
  must match the associated_data supplied for the encryption.
  + The cipertext_source's read() method is expected to return an empty bytes
  + object if the stream is already at EOF. In the case where the stream is not
  + at EOF yet but no data is available at the moment, it is expected to either
  + return None or raise BlockingIOError.
  +
  + The standard BufferedIOBase and RawIOBase base classes exhibit these
  + behaviours and are hence supported.
  +
  Returns:
  A decrypting file object wrapper around 'ciphertext_source', such that
  any bytes read from the wrapper are AEAD-decrypted using 'associated_data'
  