openocd: rewrite command 'version' as COMMAND_HANDLER

Trivial change.
While there:
- add the mandatory 'usage' field;
- document the optional parameter 'git';
- reword the documentation.

Change-Id: I6be4d4423128fa026a62e2ef355f77b69d50397e
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7488
Tested-by: jenkins

doc/openocd.texi

@@ -9101,8 +9101,10 @@ format. Optional @option{start} and @option{end} parameters allow to
 limit the address range.
 @end deffn
 
-@deffn {Command} {version}
-Displays a string identifying the version of this OpenOCD server.
+@deffn {Command} {version} [git]
+Returns a string identifying the version of this OpenOCD server.
+With option @option{git}, it returns the git version obtained at compile time
+through ``git describe''.
 @end deffn
 
 @deffn {Command} {virt2phys} virtual_address