Byte stream writer should document the atomic write guarantee

[claudiofahey]

Problem description
The byte stream writer writes buffers less than or equal to 8 MiB atomically. However, this critical feature is not documented. It should be documented so that users can safely utilize this guarantee without worrying about breaking implementation changes in the future.

Problem location
byte_stream.rs ByteStreamWriter::write.

Suggestions for an improvement

Add the following, assuming that it is correct.

If buf length is less than or equal to 8 MiB, the entire buffer will be written atomically. If buf length is greater than 8 MiB, only the first 8 MiB will be written, and it will be written atomically. In either case, the actual number of bytes written will be returned.

[tkaitchuck]

More to the point: The number of bytes written is returned. Those bytes will be persisted atomically. (Though perhaps not before the method returns.)

[Tristan1900]

Thanks for the issue, it has been added to the Rust doc now.
https://pravega.github.io/pravega-client-rust/doc/pravega_client/byte/struct.ByteWriter.html#atomicity