net: relax buf size requirements for mrg buffers

VIRTIO-84 added a requirement that each buffer is > header size but
there's no reason to require that for merge-able buffers since data can
be placed in follow-up buffers.

The current wording seems confusing enough that it seems likely
no one relies on this.

VIRTIO-159

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Message-Id: <1490654778-11857-1-git-send-email-mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Cornelia Huck <cohuck@redhat.com>

content.tex

@@ -3524,7 +3524,7 @@ \subsubsection{Setting Up Receive Buffers}\label{sec:Device Types / Network Devi
       with buffers of at least 1526 bytes.
   \end{itemize}
 \item If VIRTIO_NET_F_MRG_RXBUF is negotiated, each buffer MUST be at
-  greater than the size of the struct virtio_net_hdr.
+least the size of the struct virtio_net_hdr.
 \end{itemize}
 
 \begin{note}