Rename the DIAGNOSTICS section to RETURN VALUES and describe the

return values a little more.  Specifically, mention that a return
of 0 from NgRecvData() and NgRecvMsg() means the socket has been
closed.

Suggested by: jkh

lib/libnetgraph/netgraph.3

@@ -255,12 +255,18 @@ and
 .Xr ng_socket 4
 KLD modules must have been loaded via
 .Xr kldload 8 .
-.Sh DIAGNOSTICS
-All functions except
+.Sh RETURN VALUES
 .Fn NgSetDebug
-and
+returns the previous debug setting.
 .Fn NgSetErrLog
-return -1 if there was an error and set errno accordingly.
+has no return value.
+All other functions return -1 if there was an error and set errno
+accordingly.
+A return value of zero from
+.Fn NgRecvMsg
+or
+.Fn NgRecvData
+indicates that the netgraph socket has been closed.
 .Pp
 For
 .Fn NgSendAsciiMsg