Improve documentation about binary

[Totodore]

Hi,

While implementing a msgpack parser for the rust server implementation. I discovered (if I understood this correctly), that:

• The attachment count in msgpack for binary is never sent.
• The packet type is always EVENT or ACK and never BINARY_EVENT or BINARY_ACK even if there are binaries in the payload.

These information are quite important and also having a documentation about this would secure breaking changes between two versions of this parser (it would break compatibility with my version then).

Could you confirm the two assumptions above @darrachequesne ?