virtio-msg: clarify max size table vs recommended cap

The revision table shows the protocol-defined maximum message size,
while the text recommends advertising no more than 264 bytes. Add a
sentence to distinguish the protocol maximum from the recommended
advertised maximum.

Signed-off-by: Bertrand Marquis <bertrand.marquis@arm.com>

Author: bertrand-marquis

transport-msg.tex

@@ -220,6 +220,9 @@ \subsubsection{Transport Revisions and Maximum Message Size}
 \hline
 \end{tabular}
 
+The table reflects the protocol-defined maximum size; the recommended
+advertised maximum remains 264 bytes for interoperability.
+
 Note that a change in the virtio standard does not necessarily
 correspond to a change in the virtio-msg transport revision.