Add additional annotations to soreceive(), documenting the effects of
locking on 'nextrecord' and concerns regarding potentially inconsistent
or stale use of socket buffer or stack fields if they aren't carefully
synchronized whenever the socket buffer mutex is released. Document
that the high-level sblock() prevents races against other readers on
the socket.

Also document the 'type' logic as to how soreceive() guarantees that
it will only return one of normal data or inline out-of-band data.