lib/cmdline: adjust a few comments to fix kernel-doc -Wreturn warnings

lib/cmdline: adjust a few comments to fix kernel-doc -Wreturn warnings

Fix 'get_option()', 'memparse()' and 'parse_option_str()' comments to
match the commonly used style as suggested by kernel-doc -Wreturn.

Link: https://lore.kernel.org/20260519172259.908980-6-dmantipov@yandex.ru
Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Cc: Albert Ou <aou@eecs.berkeley.edu>
Cc: Alexandre Ghiti <alex@ghiti.fr>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Charlie Jenkins <thecharlesjenkins@gmail.com>
Cc: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>