README.devices: discuss conn= and USB/TCP/VXI in addition to RS232

The conn= specification is not strictly related to RS232 communication.
Provide examples for other USB specs (as they are used for some DMM and
LCR drivers), as well as TCP and VXI (which are often found with MSO
drivers).

README.devices

@@ -139,7 +139,8 @@ Example:
 
  $ sigrok-cli --driver <somedriver>:conn=/dev/ttyUSB0 ...
 
-The following drivers/devices require a serial port specification:
+The following drivers/devices require a serial port specification. Some of
+the drivers implement a default for the connection.
 
  - agilent-dmm
  - appa-55ii
@@ -184,6 +185,20 @@ The following drivers/devices do not require a serial port specification:
  - yokogawa-dlm (USBTMC or TCP)
  - zeroplus-logic-cube
 
+Beyond strict serial communication over COM ports (e.g. /dev/ttyUSB0), the
+conn= property can also address specific USB devices, as well as specify TCP
+or VXI communication parameters. See these examples:
+
+ $ sigrok-cli --driver <somedriver>:conn=<vid>.<pid> ...
+ $ sigrok-cli --driver <somedriver>:conn=tcp-raw/<ipaddr>/<port> ...
+ $ sigrok-cli --driver <somedriver>:conn=vxi/<ipaddr> ...
+
+The following drivers/devices accept network communication parameters:
+
+ - hameg-hmo
+ - rigol-ds
+ - yokogawa-dlm
+
 
 Specifying serial port parameters
 ---------------------------------