ACPICA: getopt: Comment update, no functional change

ACPICA commit 0d784a90bc3aac75227c4459c3553de18b9ebe7a

Document one of the option string operators.

Link: https://github.com/acpica/acpica/commit/0d784a90


Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Lv Zheng <lv.zheng@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>