docs: Update phrase on message lengths in the protocol

docs: Update phrase on message lengths in the protocol

The reasoning for why all the message formats are parseable without
the explicit message length field is anachronistic; the real reason is
that protocol version 2 did not have a message length field. There's
nothing wrong with relying on the message length, like we do in the
CopyData messags, even though it often still makes sense to have
length fields for individual parts in messages.

Discussion: https://www.postgresql.org/message-id/02a4eed2-98f0-4796-9d4f-12128ff44fe0@iki.fi