Update websocket-protocols documentation to reflect implementation

[ark-1]

The doc didn't state that a WebSocket message has to start with the number of offsets, while the implementation was relying on that:

jupyter_server/jupyter_server/services/kernels/connection/base.py

Lines 54 to 61 in 2cf8ac5

	"""deserialize a message from a binary blog
	Header:
	4 bytes: number of msg parts (nbufs) as 32b int
	4 * nbufs bytes: offset for each buffer as integer as 32b int
	Offsets are from the start of the buffer, including the header.

[ark-1]

It seems I don't have the permissions to add the documentation label.

[Zsailer]

Thanks!