User guide update

doc/user_guide/en/wspr.adoc

@@ -65,12 +65,18 @@ starts. This feature can be used to activate an automatic antenna
 tuner (ATU) to tune a multi-band antenna to the newly selected band.
 
 - Depending on your station and antenna setup, band changes might
-require other switching besides retuning your radio. To make this
+require other switching besides retuning your radio.  To make this
 possible in an automated way, whenever _WSJT-X_ executes a successful
-band-change command to a CAT-controlled radio, it looks for a file
-named `user_hardware.bat`, `user_hardware.cmd`, `user_hardware.exe`,
-or `user_hardware` in the working directory. If one of these is found,
-_WSJT-X_ tries to execute the command
+band-change command to a CAT-controlled radio, it looks for an
+executable file or script named `user_hardware`. This is done using
+`CMD /C user_hardware <band>` on Windows, or `/bin/sh -c user_hardware
+<band>` on other platforms, where band is described below. On Windows
+the first file with any extension listed on the PATHEXT environment
+variable added to the file name root `user_hardware`, and found in the
+directories listed on the PATH environment variable will be executed.
+On other platforms, the first executable script, or program, named
+`user_hardware` found in a directory listed on the PATH environment
+variable will be executed.
 
  user_hardware nnn 
 
@@ -78,6 +84,11 @@ _WSJT-X_ tries to execute the command
 meters. You must write your own program, script, or batch file to do
 the necessary switching at your station.
 
+IMPORTANT: The use of the PATH (and PATHEXT on Windows) environment
+variables is a new feature. To emulate previous behavior make sure
+that the location of your user_hardware script or program is on the
+PATH environment variable used by _WSJT-X_.
+
 The following screen shot is an example of WSPR operation with
 band hopping enabled: