mitsub_vfd docs: add point we are not using MODBUS

Author: c-morley

docs/man/man1/mitsub_vfd.1

@@ -4,7 +4,8 @@
 .SH NAME
 mitsub_vfd \- HAL userspace component for Mitsubishi A500 F500
 E500 A500 D700 E700 F700-series VFDs (others may work)
-
+This uses the COMPUTER LINK protocol _not_ MODBUS.
+The connection is made through the PU connector.
 
 .SH SYNOPSIS
 
@@ -80,6 +81,10 @@ no wait time is added to the serial data frame (don't change)
 .br
 don't change
 
+\fBPARAMETER 549\fR communication protocol - 0
+.br
+computer link protocol - don't change - (not all VFDs have this)
+
 .SH
 
 This driver assumes certain other VFD settings: