docs: Update phrase that talks about 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
new variable-length definitions of CancelRequest and BackendKeyData.,
even though it often still makes sense to have length fields for
individual parts in messages.

Author: hlinnaka

doc/src/sgml/protocol.sgml

@@ -3650,10 +3650,10 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
    indicate that it can be sent by a frontend (F), a backend (B), or both
    (F & B).
    Notice that although each message includes a byte count at the beginning,
-   the message format is defined so that the message end can be found without
-   reference to the byte count.  This aids validity checking.  (The CopyData
-   message is an exception, because it forms part of a data stream; the contents
-   of any individual CopyData message cannot be interpretable on their own.)
+   most messages are defined so that the message end can be found without
+   reference to the byte count.  This is mostly for historical reasons, as
+   the original, now-obsolete protocol version 2 did not have an explicit length
+   field, but it also aids validity checking.
   </para>
 
   <variablelist>