doc/openocd.texi: Document add_help_text and add_usage_text

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 <[email protected]>
Reviewed-on: https://review.openocd.org/c/openocd/+/6734
Tested-by: jenkins
Reviewed-by: Antonio Borneo <[email protected]>

doc/openocd.texi

@@ -8739,6 +8739,14 @@ Requests the current target to map the specified @var{virtual_address}
 to its corresponding physical address, and displays the result.
 @end deffn
 
+@deffn {Command} {add_help_text} 'command_name' 'help-string'
+Add or replace help text on the given @var{command_name}.
+@end deffn
+
+@deffn {Command} {add_usage_text} 'command_name' 'help-string'
+Add or replace usage text on the given @var{command_name}.
+@end deffn
+
 @node Architecture and Core Commands
 @chapter Architecture and Core Commands
 @cindex Architecture Specific Commands