doc: Add 'jlink targetpower' command

Change-Id: I7e6c9e75f3c70675a3ea55fd5f0d7de1a35f2c4b
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: https://review.openocd.org/c/openocd/+/8418
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Tested-by: jenkins

doc/openocd.texi

@@ -2958,6 +2958,11 @@ Display free device internal memory.
 Set the JTAG command version to be used. Without argument, show the actual JTAG
 command version.
 @end deffn
+@deffn {Command} {jlink targetpower}  [@option{0}|@option{1}|@option{on}|@option{off}]
+Switch the 5@ V target power supply on or off.
+Without argument, show the state of the target power supply.
+The target power supply is usually connected to pin 19 of the 20-pin connector.
+@end deffn
 @deffn {Command} {jlink config}
 Display the device configuration.
 @end deffn