lib/cmdline: Update documentation to reflect behaviour

get_options() API has some tricks to optimize that may be not so obvious
to the caller. Update documentation to reflect current behaviour.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>