docs: scripts/kernel-doc: Detect absence of FILE arg

Currently, when there is no FILE argument following a switch such
as -man, -rst, or -none, kernel-doc exits with a warning from perl
(long msg folded):

    Use of uninitialized value $ARGV[0] in pattern match (m//)
    at ./scripts/kernel-doc line 438.

, which is unhelpful.

Improve the behavior by adding a check at the bottom of parsing
loop.
If the argument is absent, display help text and exit with
the code of 1 (via usage()).

Signed-off-by: Akira Yokosawa <akiyks@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/7b136049-a3ba-0eb5-8717-364d773ff914@gmail.com


[jc: reworked to fix conflict with pod patches]
Signed-off-by: Jonathan Corbet <corbet@lwn.net>