scripts: kernel-doc: improve argument handling

Right now, if one uses "--rst" instead of "-rst", it just
ignore the argument and produces a man page. Change the
logic to accept both "-cmd" and "--cmd". Also, if
"cmd" doesn't exist, print the usage information and exit.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>