openocd

Document add_help_text and add_usage_text under Genral Commands ->
Misc Commands.  The text are taken from command's help strings.

Change-Id: I2bbbee8e5faa8d9654227c304e0528f5144275e8
Signed-off-by: Yasushi SHOJI <yashi@spacecubics.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/6734


Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>