doc patch: protocol major.minor numbers in NegotiateProtocolVersion message

Hi,

This patch clarifies the format of the major and minor protocol version
numbers sent by the backend in a NegotiateProtocolVersion message. The
current wording "Newest minor protocol version supported by the server
for the major protocol version requested by the client" can be read as
indicating that the Int32 field only includes the minor protocol version
number, where in fact the backend sends an Int16 with the major protocol
version then an Int16 with the newest minor protocol version that it
supports.

Thanks,
Eric Marsden