add document for adding a new device. clean up existing document due to split machin.c

doc/devices/2M-PCB23-V3_2.md

@@ -17,8 +17,8 @@ The original web interface reports this board as hardware version V3.2
 The component placement matches [2M-PCB23-V3.1](2M-PCB23-V3_1.md): same
 positions for the flash (U8), the T7/T8/T9/T10 headers, the SFP cage and its
 LED, and the unpopulated U7/U9 footprints. Build with
-`MACHINE=KP_9000_9XHML_X_V3_2`, which uses the V3.1 machine definition and
-only differs in the reported name.
+`make MACHINE=KP_9000_9XHML_X_V3_2`, whose board file is a symlink to the V3.1
+definition and only differs in the reported name.
 
 ## Tested
 

doc/devices/2M-PCB43-V1.1.md

@@ -15,7 +15,7 @@ and sold by Mokerlink.
 The device is fully supported:
 - All 4 2.5GBASE-T RJ45 ports work at 10/100/1000/2500 Mbps
 - The SFP+ port supports 1G, 2.5G and 10G modules 
-- LEDs work with the same indiciations as the OEM firmware (use KP_9000_6XHML_X2_V1_1 in machine.h if building yourself or the corresponding pre-compiled binary)
+- LEDs work with the same indiciations as the OEM firmware (build with `make MACHINE=KP_9000_6XHML_X2_V1_1` if building yourself, or use the corresponding pre-compiled binary)
 - untested due to missing Hardware: SFP+ ports equipped with 1G or 2.5G SFPs.
 
 ### Hardware overview

doc/devices/SWTGW215AS.md

@@ -9,7 +9,7 @@
 
 ## RTLPlayground target
 
-Use machine target `MACHINE_LIANGUO_ZX_SWTGW215AS` for this device.
+Build for this device with `make MACHINE=LIANGUO_ZX_SWTGW215AS`, which selects the `machine/LIANGUO_ZX_SWTGW215AS.c` board definition.
 
 Physical hardware verification: 5x RJ45 ports + 1x SFP port. 
 Port 5 RJ45 is interfaced through a RTL8221B IC.