gdbstub: move comment for gdb_register_coprocessor

Use proper kdoc style comments for this API function.

Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-13-alex.bennee@linaro.org>